Welcome to my guide!
I'm a diamond Rengar main, playing him ONLY in toplane, with a build oriented around oneshotting and most damage output. I've played him this way since he got released back in season 2, where i originally picked him up to play him in the jungle. I quickly changed to playing him top, as i saw his potential, and have played all iterations of Rengar throughout the seasons this way. I have over 1000 ranked games played on Rengar, and closing in on 1 million mastery points on him.
This guide is centered around a harcore mentality of WIN LANE -> WIN GAME, so if you want to learn how to dominate with Rengar Top, you've clicked on the right guide!

This guide will feature most the optimal build for this playstyle, with alternative routes depending on how a game progresses.

IGNITE: Can be taken in all matchups, and is my current go to if i'm not sure what im against. It should always be taken in matchups where the enemy has a specific healing ability you want to get rid of, like: Vladimir, Volibear or Tryndamere.

EXHAUST: Can also be taken in most matchups, however after the nerfs, it is no longer as viable. Exhaust can make a trade easier, and shines against champions that rely on all-in burst, like: Kennen, Zed, Riven. Also good if the enemy has an enemy Assassin, your team lacks Exhaust against (used to be be my go to)

TELEPORT: Depends on preference, and is best used vs Tanks where you don't have much kill pressure, like: Shen, Malphite or Rammus. TP does get better with elo, as you will know how to use teleport better, and your teammates are better are reacting to it. Plus that most junglers will camp you if you don't pick TP in very high elo. If you are playing 5man premade, Teleport is advised in all matchups. After the nerf, you might not want to take TP, even in premade. Your choice. Tip: If you stand in a bush while using Teleport, your jump lingers through the teleport, and you can therefor jump to stuff for 0.25s after teleporting (so you can jump to things inside the jumprange of where you teleport to). This is very useful when teleporting to a minion in botlane, so you can jump instantly, not needing to go to a bush or use ult).

FLASH: Should be used 100% of the time! Makes you harder to gank, makes you get easier assassinations if they have an escape. Flash is also usable while in ult, which is very useful if you need to get over walls, or need a tiny bit extra reach to get to your target.

DOMINATION

**Changed to Hail of Blades in 9.2.**
Hail of Blades Now makes your next auto after your jump start right when you jump, so that it (almost) instantly applies when landing. Essentially getting an extra auto every time you jump with around max jump range.

In lane you can do, example: Jump -> Land(auto) -> Wait 0.1sec for another auto -> Q to autocancel asap -> auto
This uses all 3 hail stacks + 1 more stack from auto cancel INSANE DAMAGE

Sudden Impact is MADE for Rengar as you have crazy uptime on it.

Eyeball Collection is the only talent here that gives you more damage, so i just take it. Plus it works with your bonetooth stacks.

Relentless Hunter is taken now, since Rengar needs his ult a lot for teamfights. The movement speed from Relentless hunter didn't help much anyway, since you build youmoo's. Ultimate Hunter could also be taking for more ultimates, however movement speed feels more beneficial.

PRECISION

Triumph Saves you from dying when killing the enemy, and also helps in teamfights. Helps you clean up a lot of deaths when it's a close call.

Coup de Grace Makes you deal more damage (great) and since you are going for oneshots in this build, you also also very quickly get the bonus from the other half of this rune, nice!

RESOLVE

If you want more defense, your second tree could also be:

Bone Plating is amazing for trades in lane.

Chrysalis gives great stats and transitions into better oneshots and stacks great with your passive.


STAT SHARDS
The first two shards will always be Attack Damage. This syncronizes with your passive, and honestly makes you even more of a beast lvl 1-3.

Defendsive shard choice
The scaling health shard is not worth it unless you are facing a lot of true damage(very rare).
So either pick Armor when you are facing an AD champ,
or Magic Resist when phasing an AP champ.

Due to how some AP champs scale up, it will still be worth it to take armor shard vs. some AP champs, like Kennen(cause of the insane auto harass early on).

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Notes for other runes:

Summon Aery was nerfed and is no no longer the best choice.

Nullifying Orb can be taken if you are vs a crazy AP champ eg. Rumble/ Vladimir etc.

Scorch makes for better burst, and also helps a bit in lane. One of the better choices in the Sorcery tree, however doesn't work off of Q, so there are better choices.

Ravenous Hunter is ****ty, as both your Q and W counts as AoE abilities, and therefor heals for very little.

Cosmic Insight is a good rune, especially if taking teleport, however it's in a weird tree.

Hextech Flashtraption works with jump range and can be good. However its best used as a jungler. NO LONGER WORKS

Perfect Timing is good, however it keeps getting nerfed, and because of the tree it's in its not worth getting.

Ravenous Hunter doesn't work with Q and therefor isn't worth it.

Ingenious Hunter could be good with Tiamat and Youmuu's Ghostblade, but i don't think its worth it, since Youmuu's already has a short enough cooldown to be up for every ultimate.

Predator could very well be a good keystone, however it really doesn't cater to laning, so if you want to play with this, play jungle.

Dark Harvest has the same problem as Predator, it's a jungle rune, get over it.

Transcendence is good obviously as it gives 10% CDR, it's doesn't help in lane though, which is a real bummer.

Gathering Storm one of the best Sorcery runes, but i'm not playing it atm.

Electrocute used to be the best keystone, as it ads to your burst/oneshot. Its cooldown is quite long though, but it still helps a bit in lane. STILL GOOD(HASN'T CHANGED).

Changed from maxing W second, to E. This is better for oneshotting which i enjoy more (**** those ADC's). This helps to kill an adc even if you can't get the 100% crit on the, due to the support protecting them (this gets worse with elo). Maxing W second is still very viable, and makes you better at dueling and a bit more durable. (less burst though)

First 3 levels: Q will be taken first always. For lvl 2, you can either take W or E, if you're going to fight your target, or you're afraid of dying take W. If you know you can kill your opponent but afraid of him getting away, take E. Take the one your missing at lvl 3.

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Unseen Predator (Passive) Makes you jump from a bush onto a target. You landing on a target will count as an auto attack. This auto can be Q amplified.

BONETOOTH NECKLACE Gives you % Bonus AD. This increases as you kill more unique enemies. This also works with assists, as long as you damaged the enemy 1.5 second before they died. Get this fully stacked as early as possible. Look for picks with your ult on enemies that you haven't killed yet.

FEROCITY: You have 4 stacks of Ferocity. You will get a stack from using an ability while in combat, it doesn't matter if you hit a target or not. You will also get a stack if you leap at a target, with zero stacks. When using an ability with 4 Stacks, it will be empowered and you will also get a quick movement speed boost.

This makes for the obvious skirmish/optimal damage rotation in lane:
Jump on target -> E mid-air -> Q right after you land for an auto reset -> W (also heals if you took damage) -> into an empowered ability.
(see Combos section for more info on this)

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Savagery (Q) is an auto attack amplifier, that auto cancels, that also gives you more attackspeed on your next 2 auto attacks. When empowered it deals MORE damage, and gives you MORE attackspeed, which is for the next 5 seconds, instead of just two attacks.

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Battle Roar (W) deals AP damage in an AoE around you, and heals you for 50% of the damage you have taken the last 1.5 seconds. If Empowered it will deal a tiny bit more damage, and heal you just like the regular W. But it will also remove crowd control on you. Used for Ferocity generation, or to heal if you have just taken damage. If used correctly this can win you a lot of trades.

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Bola Strike (E) is a skillshot that deals damage and slows the target for 1.75 seconds (this is a very strong slow). If empowered this will root the target instead and deal a tiny bit more damage.

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Thrill of the Hunt (R) puts you in stealth after 2 seconds, and gives you more passive movement speed. This also makes you able to jump on a target without having to be in a bush. The stealth it puts you is a camouflage (think Evelynn stealth), where they can see you if you are close, you can use your jump range indicator to check from where they can see you from. When ulting you will get vision on the nearest target (inside the ult range), as you get closer to them they will get a pair of eyes on top of them, so they can see your are near them. If another enemy becomes the closest enemy will, the first enemy still STAY REVEALED for you. When jumping on the nearest target (the one with the mark over their head, you will deal additional damage to them, and shred their armor for the next 4 seconds.

Good overall example build:
(can be used in all games, but different choices could be better depending on game)

Doran's Blade + Health Potion

Doran's Blade -> Tiamat -> Boots -> Duskblade of Draktharr -> Mobility Boots -> Youmuu's Ghostblade -> Last Whisper(upgraded) -> Edge of Night -> Ravenous Hydra

- BUILD ORDER NOTES -

The second Doran's Blade, you build with the gold you get from first blood (which you obviously get), or just if you back with around 500g. Another Doran's will help you a lot in lane, and generally only skip it if you can build a full Tiamat.
You can even choose to get up to 4 Doran's Blade if you are having a rough lane, where you aren't backing with that much gold. The more doran's you buy, the easier lane, but the worse transition into midgame.

Your first real item will be Tiamat which is a very good first item for damage (can be comboed and clear minions so you can roam). Getting this too early can be a problem, as Tiamat makes it impossible to freeze your lane, So if you back with a lot of gold early, and you want to freeze, buy a Serrated Dirk or even a full Duskblade of Draktharr before Tiamat to delay it. **More on freezing in Laning section

Buy Boots now, and start building a Duskblade of Draktharr (Build Serrated Dirk first, and use the passive in lane). When you have Duskblade you can literally start to oneshot, this item is crazy. The proc to clear wards is really great for roaming aswell!

Now if you are ahead or even in the game finish your boots into Mobility Boots. You can do this earlier than
Duskblade of Draktharr if you opted into many Doran's Blade's.
If they have way too much hard CC, take Mercury's Treads. If they are all auto attack reliant, buy Ninja Tabi.
If you are behind in lane vs, an AD champ, you can RUSH Ninja Tabi (even before Tiamat) to be able to stay in lane.

Build a Youmuu's Ghostblade now, which is great for damage and utility. Build Caulfield's Warhammer first here, for the CDR for more ults.

Final lethality item is Edge of Night, which is awesome as the enemy team WILL try and stop you from oneshotting. The spell-shield can help vs things like Lulu Ult and Janna Ult. If these are a big problem, don't be afraid to buy this before Youmuu's Ghostblade, just to have the shield earlier in the game.

Now buy Last Whisper (and upgrade it), depending on if you have trouble getting focused instantly, or if you need damage to get through their armor items(note on which upgrade you should choose for Last Whisper below).

And lastly finish your Tiamat into Ravenous Hydra for that lifesteal.
Or if no waveclear is needed(quite rare), sell Tiamat, and buy one of the items below.


Build iterations:

If the enemies have built a lot of armor, buy a Last Whisper and upgrade it based on enemy team. You may even need to build this before finishing all 3 lethality items, if they are getting armor early. **buffed in 9.3, build this before Edge of Night if you don't need the spellshield.
( Mortal Reminder vs. a team with healing or Lord Dominik's Regards vs. all other teams)

If getting dominated by an AP champ you can opt to buy an early Hexdrinker to be able to stay in lane. Possibly upgrade it to Maw of Malmortius if their team is AP heavy.

If your team needs more tankyness, and has damage, you can build either Dead Man's Plate or Spirit Visage, depending on opponents.

When entering late game, you probably want to exchange your Mobility Boots, to either Mercury's Treads or Ninja Tabi, since you won't need the movementspeed as much as the defense. Just do this when you have excess gold.

Buying a Stopwatch before an important teamfight can literally win you the game. Do so if you think that it's needed and you have excess gold, just sell it after use, unless you wan't a Guardian Angel.

THINGS TO REMEMBER

BUY WARDS! I can't stress how much this can help both you are your team. You will always be looking for picks, so if you know where the enemies is, you know when one is separated. So if you have excess gold, buy em! And remember to use your ward trinket as much as possible as well. (Note about wards placement in Laning section).

Change your trinket to Farsight Alteration at lvl 9 if your team already got someone warding a lot. Remember to use it! You can have more of these on the map if they don't get killed. Used to scout for objectives or kill-targets in their jungle.
This ward is USABLE in ult-stealth, so use it on top of an enemy for scouting if you are unsure if your kill-target is alone, as you can only see them during ult.

You could also upgrade to Oracle Lens if your team has a problem clearing wards, however other people on your team should have taken this role, upgrade trinket to this when facing Teemo ASAP to clear shrooms.

And remember to buy a Elixir of Wrath, before fights in lategame for that extra damage and lifesteal.

Health Potion's are generally only bought in the starter, and on your first or maybe second back if it's early. After this, stop wasting gold on pots, if you need healing, use the lifesteal from Doran's Blade.


NOTES FOR OTHER ITEMS

Guardian Angel can also be a very good choice if you are fed and getting focused down, but generally it is better to keep on building damage and kill the enemy, before they kill you.

Mercurial Scimitar is actually a good item, it gives some lifesteal, which ain't bad, and the active is good, don't get me wrong. But Rengar can already cleanse with his empowered W, so i don't feel like this item is needed. Could be used vs. some teams though, if you get cc'd instantly.

Phantom Dancer is good for lategame, since attackspeed will be more worth and gives you a nice shield. Personally prefer Statikk Shiv

Statikk Shiv has some waveclear if you sold Tiamat for example, and also provides more burst, a good late game item.

Rapid Firecannon could be good if the range worked on Rengar's jump range, but sadly, it doesn't make it larger. Don't buy.

Trinity Force can be used as a last item instead of Tiamat or Boots, if you really need to oneshot an enemy, but is rarely needed.

Black Cleaver is really good, however i don't feel like using it in a burst build. Should be used in a bruiser build. Or if you want more CDR.
MIGHT WANT TO RUSH THIS AFTER Tiamat IF VERY BEHIND. Do this if you want to transition into early tank items. I would only do this if i'm very behind, and the enemy champions will be hard to assassinate.

Essence Reaver Is bad now, you don't use mana.

Stormrazor should now only be bought after other attackspeed items, which you don't buy anyway, and even if you do, you won't have a slot for this.

Infinity Edge is essential in a crit build, but not in a lethality build, sorry. Don't buy.

The Bread and Butter

Ult(or be in a brush) -> Q -> Jump on target -> E (mid-air) -> Tiamat -> W -> Land -> into an Empowered ability of choice.

If you know that the target will deal a lot of damage to you, you can delay the W and then do two W's back to back, for 100% heal of what damage you just took.

USE OF EMPOWERED ABILITY

Use Q when you need more damage, and there is no other threats, and the target is still in range. (INSANE DAMAGE)
When using empQ after the combo above, put an auto attack in-between the landing and the next Q. (unless you really need a big burst, then just spam the Q, however this is usually not ideal)

Use empW when you need a heal, or when you need to get out of CC, this is used a lot in teamfights.

Use empE if your target is getting away basically. If they have a lot of movement speed, or if they flashed.

THE TRIPLE Q

This is VERY situational, as you will only have X amount of seconds to do this once it's set up. This is by far the combo with most damage, however you wont be able to do it very often.
You stack up 4 stacks, and then instantly ult after. It takes 2 seconds for you to get into ult, and then you press Q right before you leave combat (watch your passive icon for the timer), so that you have your emp Q on you, and you then have to jump to your target before your empowered Q expires. The full combo will look like this:

4stacks -> Ult -> Q(emp) -> Jump to target -> E -> tiamat -> W -> Land -> Q -> Q(emp)

Tricky E Combo

If you start your ult with full Ferocity stacks like with the triple Q combo, and jump on a target before they deplete (very short time to do this), you can use the E first to root your target instantly. Very useful for killing champs like Ezreal or Vayne. But hard to manage.

Longer range on bush jumps

If you click on the ground towards your jump target, and then on your target you can get a tiny bit more range on your jump. Should be done if you are unsure if you will be able to do the jump. Helpful for jumping from midlane to lane minions, bush to plant in river, bush to dragon/baron etc.

Jump timer reset

Due to jumps actually being autoattacks, how fast you can jump again is dependent on attack speed. Due to Q actually resetting your auto attack (after the short dash), if you jump on something in a bush and want to jump to something else fast, you jump the the first target, and then instantly press Q while right clicking on your next target, and then you are able to jump to the next target without having to wait for an auto attack.

Q damage on towers

Earlier in my guide i stated that Q damage doesn't apply to towers... that is not entirely true. If you have Q applied to the auto attack that you jump with, and you jump to a turret(either from a bush or from ult), the Q damage WILL actually apply to the tower. The only bushes you can do this from is the second tower in top right side, and second tower bottom left side.

Flashy E

Since E has a small cast time, you are able to use Flash while it's being casted, and thereby cancelling the animation, AND changing where the bola comes from. You can even do this while casting E in a jump.
How to do it:
Put your cursor in the direction you want to cast E, press E, then flash somewhere, and your E will now be cast from your new spot, but in the same angle you used when clicking E before flashing.

Actives Tip:

Remember to use Youmuu's Ghostblade and Edge of Night before jumping on targets with ult if you one or both of them. You use these AFTER pressing ult (while you are running towards your target). Can obviously also be used without ulting if needed for different scenarios.

I highly recommend going into Practice Tool mode and trying out the different combos and tricks, so that you are ready to do them in-game.

HOW TO GET FIRSTBLOOD

When the first minions get into lane be there early and pull them towards the bushes, so that you have an easier time jumping to them. You do this by hitting the first minion once, thereby making them run to you, and then run towards the bushes(but not into).

Harass the enemy laner by going in a bush, then Jump on them and then Q . Repeat (this is lvl 1)

At lvl 2 try have 2 stacks of ferocity, then level up E or W and kill the other guy, (using the combo above, with e or w) by either rooting them with E or use your W for healing when they fight back, also use exhaust if they are going to kill you, or are getting away. Then use an empowered Q on them or if you took E, you can use empowered E if they are getting away, even after the slow from exhaust.

Also, put a ward to watch out for the enemy jungler before 2:50 min into the game.

Good ward placements throughout the laning phase (go deeper if ahead):

Beyond level 3

Whether you get first blood or not, your goal will still be to look for opportunities to get solo kills. If the lane is even, and you don't think you are about to get ganked (look at where the enemy jungler is on the map, and try to think where you think he would go next if he recalls), it is very important to maintain bush control. Try to keep the minions in the middle of the lane as much as possible, so that you can utilize the bushes. Gain stacks and do trades where you use your empowered ability on W, so that you regain all the health you lose in the trade.

- Jump on enemy, do damage with Q and E, then use W two times, and run away with the speed boost you get from empW -

From doing this once or twice, depending on opponent, you should be above the enemy in health. This is when you start to look for an all-in. Build up 3-4 stacks, and go for it, you will be surprised of how much damage. In this case you will be able to get off an empQ without the use of W(cause you jumped with a few stacks). So if he fights back, you delay your W to use it when you can regain as much health as possible.

Ahead or Behind

If you are ahead, start getting vision in the enemies jungle, a nice ward (if very ahead you can even place a control ward) to have would be at the enemies top side buff. Use this to get even more information or even better if you can kill him.
When ahead you can easily go and take the topside scuttle crab, for extra gold and vision. (jump from bush, so you can instantly use empE on the crab to remove it's armor)

Freezing
When ahead you also have another option than going aggressive, stay defendsive. Freeze your minions infront of your tower, by saving atleast 3 enemy minions from his wave, and let them kill your minions. You only lasthit, while jumping from the bush. You can essentially keep this up the whole laning phase, if you don't want to roam. And because of your jump range, the enemy won't be able to last hit a single minion, if they won't risk you jumping on them. As stated in Items section, you can't do this if you have Tiamat, so build accordingly.

If you are behind, you will have to be even more certain in which trades you make, and be even more careful of the enemy jungler, if you aren't sure you can win the trade, it's better to just stay back and farm as best as you can. Wait for a gank, or wait till your enemy leaves the lane so you can push and roam with your ult.

LANE TIP (Bush Juggling):

If the enemy follows you into the top bushes, you can kite between the bushes. Just jump on them and quickly move to another bush. Use E to slow them, then when they are almost in the bush you are in, jump to them again, and move into the brush you came from. Then repeat. Weave in Q's if you can. Can also be done without slow, but a bit harder.

Start look for more opportinuties to roam, and you can even gank bot if you backed (if you do this, ask your jungler to hold your lane if needed). This can give you easy dragons. START GETTING YOUR NECKLACE FULLY STACKED (prioritize these targets)

Mid and Late Game

You have some options: Splitpush, Teamfight, Get Picks.

Now these can all be the best, and the worst, choices in different situations, i can't tell you to do one of them every game.
But here is how you would go about them:

Splitpushing

Splitpushing can be very good for Rengar now, as he can have problems with getting to his target in a teamfight, due to how his ult works. When splitpushing you are essentially making their team split up. They will have to send someone to defend. So here is what you do: If its someone you CAN'T kill, maybe they send a Malphite down. This is when you just push in the wave and leave the lane as quick as possible, pop you ultimate and go in on their team, when they realize this, they are maybe going to split up and you can get a pick. If they don't, get your team ready and 4v5 them. And if they pull all the way back, get a Baron or Dragon.

Teamfighting

This can be difficult to do, but sometimes you have to. YOU ARE NOT THE INITIATOR. Wait for the fight to begin, then you jump on a squishy target. It's not certain that you will be able to get the bonuses on a target, as every enemy are on top of each other, so either jump on the one that is marked, and get the bonuses, or just on their carry without the bonuses. (Most of the time you just jump on their carry). Now to have the best chance of having their ADC as your nearest target, you need to FLANK. Do NOT stand with you team. Go around and behind them with your ult, and get a good jump in on their carries, that is how you win a teamfight.

Also if you manage to kill your target, search for brushes so you can keep jumping. This also helps you, as your need to wait for your cooldowns, if the fight isn't a "clean up".

Getting Picks

This can work very well, or very bad. For this you'll need vision. Put wards in their jungle, and look at their lanes and think of where they would go if they leave a lane. If their ADC is farming bot, but need to go mid, catch him in the jungle. If this happens just ult and go for it, you get vision if you already know they are the closest target.

Thank you for reading my guide, feel free to post in the comment section, about any questions or any criticism you might have of the guide.