Hi there, my name is Donutknol. I'm currently a silver 3 top/jungle main and I absolutely love rengar. I mainly play him top but won't hestitate to play him jungle either. Right now I have level 5 mastery on him and he is definitely my go to pick if either top or jungle is open. (I also play him support sometimes). Anyways, welcome to my guide, and please make sure to tell me if anything is wrong and leave a vote if you liked/disliked my guide. Please note that this guide is not done yet and there is still work in progress. There are still some builds to be added and alot of chapters to be added.

Rengar is a very strong assassin in every level of play. He is played in almost every division and was seen alot in the season 4 worlds. If played well he can snowball out of control and even when not snowballed, he can still pop carries if he's decently farmed. He can even be built tank if desired. You won't have as much damage but if your team needs it it's not bad at all. He is also very fun to play in my opinion.

Pros

1. Can easily pop carries if he has items
2. Good in all stages of the game
3. Good dueling, also against tanks
4. Can be built tanky as well as with damage
5. Alot of DPS
6. Can be played in alot of roles
7. No resources that prevent him from using abilities if he doesn't have them
8. Low cooldowns
   
   

   Cons

   1. not very strong when behind
   2. Not alot of CC
   3. Requires some knowledge of the champion to be played properly
   4. Easily shut down by CC
   5. Usually pretty squishy

Top lane

Probably the best summoner spells in the game for almost every champ. Can be used offensively as well as defensively or for positioning. Could be replaced by ghost if flash not available yet. This also applies for jungle

I usually take this now to first off not fall behind if I need to back with a wave pushing into my tower but also for the map pressure. Don't take this when you're not that experienced with Rengar yet.

When you're still a beginner as rengar this is probably the best pickup as Rengar. It helps you secure kills and snowball. When you get alot of experience you won't need this anymore to secure kills so you can switch it for Teleport.

Jungle

Absolutely mandatory for junglers. With the new jungle it's pretty much impossible to clear without it as rengar. Also used to secure objectives like Baron Nashor and Dragon.

Other options

This can be taken instead of Flash for increased sticking potential, although it's usually inferior.

If you know you are against someone with high burst like a Zed or Lissandra this is an option. This gives you amazing 1v1 potential.

Starting Items

Top lane

Your usual start. It grants you almost everything you could want: Extra damage, some health, and some lifesteal. It is an aggressive item, which fits rengar perfectly.
For some health back when you've taken damage. This can save your life, so do not hestitate to use it when under 50% hp. Same applies to jungle.

Jungle

Mandatory for junglers. It grants you damage against the monsters, some regen when getting attacked by jungle monsters and it grants you extra gold for each large jungle monster kill.

First core

Top lane

This is an absolute necessity on rengar. It grants you waveclear that you need, alot of AD, burst and some decent regen. Get this as first item.
This item is almost as big of a necessity on rengar as Tiamat. It gives you AD, cooldown reduction, crit chance and an active which gives you movement speed and attack speed, which is great in 1v1's or when trying to pick someone off in your ult.
Get back to lane faster, or to other lanes if you're roaming. Also helps when running or chasing. Definitely worth it for only 325 gold.

Jungle

This is as far as you're going to go into the jungle items. It gives you an AoE smite that stuns monsters and gives you health (and mana, which doesn't apply on Rengar) back based on how much you are missing.
Pretty much the same as in top lane, gives you great stats and better jungler clear.
Again, same as in top: Very good stats, better ganking with the move/attack speed.
You get around the map faster, be it from lane to lane, from jungle to lane, or from camp to camp. It saves you alot of time together with the movespeed from your bonetooth necklace. Even more necessary for jungle than for top.

Mid game core items

This is a really good item on Rengar. It gives you a ton of cdr which translates into more dps and more ults, so more kills/escapes.
This gives you alot of sustain with the active and your basic attacks. It also grants you alot of AD. Especially in the jungle this is really nice to have since you can sustain with this instead of Empowered Battle Roar so you can potentially gank more or clear faster by using Empowered Savagery instead.
I've explained this already for top as well as jungle.
This gives you some decent AD, but the most important thing about this item is its passive: You ignore 35% of your opponents armor. This gives you alot of damage even against tanks because 35% of their armor pretty much doesn't exist to you. Also helps killing carries with armor items such as Zhonya's Hourglass or Guardian Angel.

Late game items

This grants a load of AD, some crit chance, and of course, what makes this item special: your critical strikes deal 2.5 times your AD instead of just double. This build has alot of crit so this amplifier is really nice to have.
Usually an item seen on AD carries, it gives alot of crit chance, attack speed and movespeed. It also makes you able to move through units. Really nice with this build because of the crit and the attack speed helps in prolonged duels (although they won't be that long by the time you get this) and also helps with killing objectives such as baron and dragon. With 2 of these you will have 100% crit chance, so a pop on a squishy is almost garuanteed. You can sell your boots for a 2nd one of these. It will decrease your movespeed by a bit but you will still have over 400 movement speed without any actives or abilities.
An alternative to a 2nd phantomdancer. It grants you alot of different stat, and the only stat Rengar doesn't use at all is the mana. The stats you care about though are the health AD, attack speed, crit chance, movespeed and spellblade. The spellblade is the main part that makes this a viable replacement in the sense of burst. It amplifies your next basic attack based on your base AD. With this you will have 80% crit instead of the 100% with 2 phantom dancers, so your burst is a bit less reliable, but even if you don't crit, you will have alot of attack speed and can still kill a carry very quickly. This will actually give you a bit more tankiness and towerpushing though.

The infamous Rengar burst combo. Because of this you can pop almost every carry instantly. It requires some practice, but once you have learned it it's definitely worth it. This is how you do it: First off, make sure you have 5 ferocity. Then, use your Thrill of the Hunt. Once you see the carry you want to kill and are close, activate Youmuu's Ghostblade, activate Empowered Savagery before you jump. Use your Bola Strike, Ravenous Hydra active and Battle Roar mid-air. If they're not dead at this point, you got alot of attack and movement speed from your Empowered Savagery, Thrill of the Hunt and Youmuu's Ghostblade so you can easily run after them and attack them some more before getting out or continuing the fight. If you are trying to kill a champion with high mobility that could survive your burst use Empowered Bola Strike instead of Empowered Savagery and use standard Savagery when you land on them. This is riskier than the normal combo because there is more time to react for the enemies.

Early game

Top Lane

Unless against a counter, play aggressive. Stay near the bushes and jump on everything. If you can jump on your enemy laner and q after you landed for a good trade. If you have 5 ferocity also use empowered q on them. Once you hit level 2 start looking for good bola's. If you can jump on them/a minion near them, snare them with empowered e and get some hits off. Remember that if you want to secure a kill using empowered e is usually better than q because they won't be able to escape. Also remember that it's not horrible to use ult to get away, as long as you don't lose a lead/fall behind. Also never forget to farm: If you are 2/0 but they have 50 more farm than you they will still be ahead in farm, since around 15 creeps is equal in gold to a kill.

Jungle

In the early game you usually just farm. You don't have any great ganking tools except for empowered e. Start the camp on the side of your bot lane, either gromp or krugs, because you need a decent leash. Then take the buff nearest to your first camp and then take scuttle crab. Scuttle crab will give you quite a bit of gold and health, since like all big monsters aside from buffs, it gives you a % of missing health (and mana) back when you kill it, and since it doesn't attack you you will gain health. Then go to your next buff, kill the other scuttle if up and then either another camp or see if a lane is overextended: if not, keep farming your jungle until level 6. As soon as you hit level 6 you can start ganking well: Make sure you have 5 ferocity before you gank. You can gank from the river or through the lane, but make sure your lanes knows you're coming: ult, run towards the enemies and when in range, jump while using empowered e and then just kill them if possible. Also remember that you can get dragon if you get kills on bot lane.

Mid-game

Top lane

By now, you should start roaming. Try getting a kill in mid lane or collapse on their jungler with your mid and/or jungler. If bot lane is behind you can also roam there, it will be worth it because there are 2 potential kills there, and 1 of them is almost always very squishy, so easy to pop with your current build. If your entire team is winning, focus on farming and bullying your lane opponent.

Jungle

At this point you should have your ult on a decently low cooldown, and use it whenever you can. Focus on bot lane to get dragon control, since this is a really important objective, especially in late game, since 5-dragon buff is arguably stronger than a baron buff in fights (baron is better at pushing and sieging).

Late game

At this point, your role as jungler and top lane is exactly the same: Kill the enemy carry. Try to have 5 ferocity as much as possible. If you do not have your ult you need to play way safer. Tell your team not to fight until your ult is up, since it is semi-necessary to assassinating a carry, unless you are fighting in the jungle. If you have killed one/both of their carries continue being a rampaging kitty (such pun, much wow). Also focus on getting towers and of course baron and dragon. Never underestimate dragon buff since the true damage and all the buffs are really insane, since your Bola Strike can do over 600 damage with the extra AD and the true damage over time. Also do not underestimate how fast you can take baron/dragon with your team. If you have 100% crit, use empowered q and are not tanking baron (baron halves your damage) you will shred baron so fast. Add a cassiopeia and a jinx to your team and you will kill baron within 10 seconds. You can also easily solo dragon since it has way less health and doesn't reduce your damage. IF you solo drag though, tell your team to be careful because they might get engaged on. Keep your opponent's teamcomp in mind with this as well: If the enemy team has a Sejuani with Turbo Chemtank it will be very easy for the enemy to take that fight with you not there and win it.

28/6/15: Guide published
30/6/15: Playstyle/positioning and bruiser rengar top added.