Welcome to havoc!
Renekton is currently one of my favorite champions and i've played him for quite a while now.I have learned a lot,and so i decided to share what do i know about him with everyone else!
So let's start it,shall we?To shortly describe Renekton : He is an AD Offtank with possibly the best early game among all top laners,but with one of the worst late games of all top laners.Im not saying it sucks,other top laners just scale better.
Renekton is not a very difficult champion to play,but to master him the player has to know his possibilities and how to use his passive.Once you're done with that,you will win top lane regardless of the matchup.
How do i we do that? You shall know with my guide!
Also,feel free to leave positive and negative feedback ; Just be serious with it,i want to fix the mistakes i make ASAP.

Pros + Insane early game sustain with Cull the Meek and ridiculous damage with an empowered Ruthless Predator + Potential to kill 2 enemies in a 2v1 gank thanks to Reign of Anger and Dominus + Great wave clear + Ruthless Predator is a great tool for peeling or locking down enemies + Can handle most melee champions at top lane even the best ones (like Irelia or Jax), with very few exceptions. ( Mordekaiser and Fiora for an example.) + Very dangerous to tower dive + Slice and Dice Makes him a very safe tower diver + Ruthless Predator resets tha autoattack timer (Which is more meaningful than you might think) + Has a very reliable stun + One of the best laning phases in the game + Hes an alligator... Or crocodile... God dammit!

The gator is a god! Tl;Dr A stun,great waveclear,insane sustain,tough to kill even with your jungler,dangerous to dive,while remaining a good tower diver. Renekton has "Insane early game" written all over him.

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...For some time We all know one thing : No champion is perfect,and Renekton is not an exception. Renekton struggles against crowd control and kiting really hard.Also,all the power Renekton could have in the late game is shifted into his early game,making his late game really mediocre.

Cons - Easily denied by CC - Only one,melee range CC ability - Recent popularity of his hard counters ( Cassiopeia, Lissandra, Gnar etc.) - Most top laners outscale him on levels 11+ if the gator didn't make their lifes enough of a nightmare - Nasus hates him - Can't be played passively since it ruins his early potential

Our masteries support agression, and defiance in the face of death and crowd control. Damage, survivability, being unstoppable - Everything you need to play this rampaging reptile, and at the same time, everything this mastery set provides.
I bet you expected some detailed informations on the masteries, huh?
Well, you will be able to see those here once I am done experimenting and being lazy. Patience.

Movement speed quints, Armor yellows, Attack damage reds. And that's how we roll as the lizard.

As for the blues, we take 3 flat magic resist, and 6 flat cdr. "Why's that?" You may ask.
Personal preference. I like to have that guaranteed 10% cdr throughout the whole match.

Also, 1% crit chance in reds. Well, trading 1 ad for 1% crit chance is efficient as hell. I have been testing those out for a while, and after getting first blood twice because of a lucky crit, i am using them all the time. Just remember : Sometimes you might get yourself a kill just because of a crit, or sometimes you might not crit for the entire game. You can't really rely on this because... You have a 1% chance to crit, dammit.
Pro tip: Before playing a game with 1% chance runes, pray to the RNG gods. This should help most of the time.

I used to take flat AD Quints, but i switched to movement speed. The answer to the question "Why?" sounds something like this : The new Black Cleaver shredded the idea of using the almgihty Brutalizer on Renekton, because it just doesn't build into anything good. HOWEVER, the Black Cleaver itself gives the alligator some better utility now (Movement speed, more CDR), in exchange for 10 less AD. This makes Renekton's late game with the Black Cleaver stronger than ever, because come on, that much utility > 10 AD. This doesn't answer the question at all, so I'm just going to say it like this ; the 6 AD makes no real difference as Renekton early on, and neither does it impact the late game, unlike the 4.5% movement speed increase, which is about 16-20-something mvoement speed throughout the game. Being slightly faster than an average champion often gives you the edge, since you can either catch up to someone, or get out of a bad situation alive. Done.

Reign of Anger
Renekton's abilities have no cost,which is the first great thing about the passive.Instead of mana,his resource bar is Fury which goes up to 100,increasing by 5 when renekton deals damage with a basic attack,and begins to go down when Renekton is not fighting for 8 seconds.When Renekton has at least 50 fury,his next use of Cull the Meek, Ruthless Predator or Dice will consume 50 fury and have a powerful bonus effect,which is the second amazing thing about this passive.Those buffs are :
Increased damage and healing from Cull the Meek.
Increased damage,a longer stun and 1 more strike from Ruthless Predator.
Increased damage and an armor shred for Dice.
It's Renekton's main power source.Without using it properly,you might as well go back to playing teemo.

Cull the Meek
Renekton slices all enemies around him with his...Weapon thingy,dealing damage and healing himself for a value that scales with your AD.
Sounds pretty cool,eh?But what is the cost of it?None!You can just spam it over and over again and constantly have a good amount of health.This ability also prevents your lane from being pushed too hard,but can also push the enemy lane too hard so be cautious when using it.Also,note that Renekton gains fury for EACH ENEMY HIT with this ability, making this his best tool to get mad.

Ruthless Predator
Renekton's next basic attack deals increased damage and stuns his target.Also,damage from this ability is given out in 2-3 attacks,each applying on-hit effects.(3 attacks when empowered by Reign of Anger)
This is the ability that makes Renekton's early damage.When used at 50+ fury,it deals 150+ damage on the very first levels.Prioritize using your fury to empower Ruthless Predator in duels.A single empowered use of it can turn the tide of a skirmish on your side.
Also, in the lates patches, Renekton got some important changes to his fury. The most important change for me is the one that affects the laning phase the most : An un-empowered Ruthless Predator on an enemy champion generates 20 fury. 20 DAMN FURY. You can now try out some safer combos than before, like getting 30 fury, dashing in on the enemy laner, hitting him with a Ruthless Predator and then using an empowered Cull the Meek.

Slice and Dice
Renekton dashes a short distance,dealing damage to all enemies along the way.If Renekton hits an enemy with it,this ability may be cast again.
Pretty simple,eh?There is just one thing to say about it : Make sure not to use your fury to empower Dice when you want to charge at an enemy to strike him with an empowered Ruthless Predator.How to do it?Info in section "Laning".

Dominus
Renekton literally goes big for 15 seconds.During this effect,he has more health,deals damage to enemies surrounding him each second and most importantly : Passively generates fury.
Just take off your pants,press R and go wild.Who wouldn't fear a rampaging alligator?

Reign of Anger - The passive that makes Renekton's abilities extremely powerful.To master the alligator,you must know how to use his passive with most efficiency.Details on how to do it in the "Laning" section.

Cull the Meek - Start with it only when you are against a ranged opponent.You will not get to use Ruthless Predator against a ranged opponent on level one,and Slice and Dice would only serve the purpose of falling back a bit,which doesn't make it a good choice.
Regardless of the matchup,we want to max Cull the Meek first for it's great aoe and sustain.

Ruthless Predator - When against a melee champion,start with Ruthless Predator to prevent enemy agression.A stun with some good damage will ensure you win the trade in most situations.Also,remember that Ruthless Predator resets your autoattack timer so try to use it after landing a basic attack.We max Ruthless Predator last,since it scales horribly. There is barely any improvement between the levels of this ability. Slice and Dice however ; It's damage scales with level better, it's cooldown is reduced, it's a dash and it's aoe. So yeah, max the W last.

Slice and Dice - Renekton's tool of lane dominance.Dash a long distance through the minions,and proceed to destroy the enemy's health bar.Not much to say about it.
When running away,make sure to save Slice and Dice for the minions so you get the second dash.(If there are minions nearby of course.)
Also,if you have to run away from an enemy gank for some reason,wait for the jungler to use his gap closer on you,and then use Slice and Dice to hit him for the first dash,and then dash a second time.Like i said : Max Slice and Dice second due to how well it scales.

Dominus - Level it up whenever possible.Once you have it,you can most likely fight enemies 1v2 and end up killing at least one of them and the second one fleeing.

Tiamat / Ravenous Hydra
The item you want to buy in pretty much every single game as Renekton. If you start the game by rushing a sunfire cape, a Tiamat should be your next target, since its just a brilliant item. Pushing power, aoe, health regen and burst all in one item. That's nothing but poetry. If you get an early lead, rush a Tiamat, because if you recall with a lot of gold, purchasing a Tiamat will give you more than a brutalizer and X item component you could also buy.
A very important thing about the active of both of those items : Use it when you're casting your W. Ruthless Predator has a long animation, making you disabled for a long part of the stun on your enemy, but the "Crescent" fixes this. Using Tiamat or Ravenous Hydra cancels your W's animation and sets you free. Once you get either of those, Dash in on your enemy laner, stun him with Ruthless Predator, use crescent as quickly as possible and then Q. This should absolutely DESTROY 40-60% of that enemy's health bar. Maybe about 20-30% if the enemy is a tank.

Spirit Visage
Spirit visage is a pretty core defensive item,because it gives MR,health regen and increased healing which will make Cull the Meek more effective.Make sure to take it when the enemy team has at least 1 champion that deals magic damage,even if its a suport.Really,the MR is just an icing on the cake.

Sunfire Aegis
I personally love this item because of it's aura magic damage,it's like a permanent ult...Or at least it's damage aura. An item beyond epic that you can rush in certain matchups. It should be your core armor item.

Banshee's Veil
A very situational item,pick it up if the enemy team has a champion that deals crazy magic damage to you and you need more health.

Last Whisper
Is the enemy laner a b...astard and stacks armor just for you?If you are really desperate to kill,you can take the last whisper and show him who's the god and who's the guy who just spams armor.

Blade of the Ruined King
The item that is actually better than anyone thinks.So,let me explain everything :
Botrk's passive deals 8% of the enemy's current health as physical damnage every time you strike him with a basic attack.
Ruthless Predator strikes the enemy 2 (or 3 times if you have 50 fury),each time applying on-hit effects.So if you have botrk,you will deal 16-24% of the enemy's current health as damage on top of Ruthless Predator's damage,which is insane.
A really delightful choice against enemies that have a lot of health.You can also build it whether the enemy stacks health or not just to deal more damage.

Morellonomicon
HARRY
YOU
ARE A LIZARD

Wit's End
Remember when i said that Ruthless Predator applies on-hit effects 2-3 times?Yeah.
Wit's end can be a good option if the enemy is buying armor against you and you need MR.
Purchasing Last Whisper is not always a good option against armor.It also feels good to have some attack speed on Renekton :).

Must have :
Flash doesn't even need any explaination. Let's be real,it doesn't. Nah it doesn't. It doesn't.

Ignite If you are the type of person who prefers tormenting a single enemy over map pressure and surprise christmas parties in summer, this spell is for you, and Teleport clearly isn't.

Teleport is an amazing spell to keep your lane opponent away from your turret, or either to do a shurimian inquisition in a skirmish or a teamfight. Unlike Ignite ; you can take teleport in every matchup and it will work splendidly. However you must always take it against champions who either require a lot of your attention ( Tryndamere, Nasus etc.), or are difficult to kill because they have more poke and range (so basically most ranged top laners).

No,just no :
Barrier?Sersiously?What coward takes Barrier on a lane different from bot lane?

Heal - ONLY COWARDS FEAR DEATH!

Exhaust - Even Phreak didn't ever speak a word about Renekton support, so neither should you.

Revive We miss you my friend, we miss you...

Clarity HARRY, YOU'RE A LIZARD!

Try to find a balance between farming and ravaging the enemy top laner if hes melee.If you are against a ranged opponent,just farm and wait for either a gank or a mistake from that enemy.

Now,the most important thing : Using Reign of Anger
Farming is just a preparation for engaging on the enemy,but to engage on the enemy early on,you must have your fury ready to empower Ruthless Predator because of it's insane damage.To do that,get yourself to 45-49 fury,dash twice through the minions,hit the enemy champion with a basic attack.This will give you 50+ fury.After that,instantly use Ruthless Predator to reset your autoattack timer and take a big chunk of the enemy's health.This combo is the most important thing in your offense in lane.

Now,when to empower Cull the Meek.
In a duel,you always want to use your fury on Ruthless Predator,unless you have 100 of it and you already used it.
There is an exception of it : When you engage on the enemy in a group of minions,wait to take some damage.If the minions will win the fight for that enemy,he will get confident and he will think that he can kill you.Then,you give him the plot twist : You use the empowered Cull the Meek to kill the minions or at least damage them and also to damage the enemy.This will also heal A LOT of your health,most likely turning the fight to your side.

Yeah...I think that's it...

When the enemy jungler comes to gank you,don't freak out.If you want to run away,dash through him with Slice and dash a second time with Dice.Don't waste the ability to dash only once.
But note that you do that only when both enemies have full health or one of them is fed.Im pretty sure that an empowered Ruthless Predator will win the fight for you.Post-6 it will be even easier for you to kill them,but remember that there is one exception : Vi.
Vi has a really solid amount of damage and her ult is just...Ugh.
Don't go into a 1v2 scenario against a Vi,unless one of the enemis is low on health or you are fed.Really,Vi is like the scariest jungler for me. :/

Your job is really simple :
1.Take off your pants
2.Press R
3.Jump into the enemy team
4.Proceed to annihilate them
But remember to go back and save your carry from some mean enemy that ignores everything and just tries to kill him.The carry is more important than you (sadly),and you have to protect him.Remember to prioritize using Ruthless Predator on that bastard enemy and not on some random dude that is easy to kill.This one stun can win you a whole teamfight.

Is Renekton an alligator or a crocodile?
Nasus and Renekton are based off egyptian gods : Anubis,and Sobek. ( Azir is possibly based off Ra)
Anubis was presented as a man with dog's head,and Sobek was a man with a CROCODILE's head.
This POSSIBLY means that Renekton is a crocodile,but "A Wild Crocodile" is too long for a nickname so he will just remain an alligator to me.

Random Screenshots

This is how laning against a Trundle looks like.
And that's my CS record folks. (Normal game, but still)