Pros

+ Correct Sustain + Huge damage + Strong mobility + Good farmer

Renekton is one of the best toplaners throughout season 3. Almost nobody can handle him early game, and his potential of snowballing is huge. Also, Renekton has one of the strongest kit available for a toplaner : His Cull the Meek provides him great sustain and AoE damage, Ruthless Predator provides huge damage and a bit of CC, Slice and Dice makes him able to escape easily or to chase easily, and Dominus bring the tanking he needs while dealing additional damage. His passive, Reign of Anger allows him to regain the advantage in a trade, which make him imbalanced early.

Cons

-- Falls off late game - Very item-dependent - Weak if he looses lane

Renekton has also one big weakness : His late game. He will be pretty much useless since he won't be able to tank properly or to make huge damage depending on how you will build him. But in either ways, he will be useless past mid-game. He also relies a lot on his core items such as Sunfire Aegis/ Spirit Visage/ Black Cleaver etc. If he can't build them fast enough, he will be useless (E.G : When he looses lane.)When playing him, do not go for the late game and end it ASAP.

• Greater Quintessence of Attack Damage: Renek has huge AD ratios, let's use it.
• Greater Mark of Attack Damage: Renek has huge AD ratios, let's abuse it.
• Greater Glyph of Magic Resist: Since your lane domination is awesome early you will be roaming sometimes, flat magic resist will help you tanking more magic damage when ganking midlane.
• Greater Seal of Armor: Globally makes you tankier.

• 9 points into Weapon Expertise : Combined with Slice and Dice, it gives insane armor penetration to help you deal more damage.
  
• 21 points into Honor Guard with Block and Unyielding because you won't really need a lot of CC-Reduction with Renekton (thanks to your Slice and Dice), and you will be taking a lot of autoattacks.

• 9 points into Veteran's Scars to have a bit of tankyness.
  
• 21 points into Executioner with Weapon Expertise and Havoc since your main goal will be to deal the most damage possible.

Reign of Anger
Renekton gains fury for every autoattack he makes. This fury can empower his abilities with bonus effects.
This passive make trades easier than ever : It adds additional damage to your abilities and you gain it faster while on Dominus or when you're low life. Use it with Ruthless Predator early to help you dominate the lane.
Cull the Meek
Renekton swings his blade, dealing physical damage to nearby enemies and gains a percentage of its damage as health.
This is your main source of damage. It is also your main source of sustain.
Ruthless Predator
Renekton's next attack strikes twice, dealing physical damage and stuns for 0.75 seconds.
This is a little stun with huge damage, early game if you manage to stun your opponent with the 50 fury bonus you'll bring him mid life. Remember it has a huge AD ratio and cooldown, so use it wisely !
Slice and Dice
Renekton dashes, slicing for physical damage. Hitting an enemy grants the ability to use Dice for 4 seconds.
This ability offers a nice escape or chasing potential for Renekton. You also will be able to reduce the enemy armor if you use dice with 50 fury, which is good to peel some targets with high armor ( Jarvan IV)
Dominus
Renekton surrounds himself with dark energies for 15 seconds, gaining health. While active, he deals magic damage to nearby enemies and gains 5 fury per second.
This will bring huge magic damage combined with Sunfire Aegis and additional fury to help you deal more damage temporarily. You can also use it if you're getting very low and you don't want to die, since it gives enormous amount of HP.

Armor

Magic Resist

Damage

Winning lane as Renekton is simple, Basically you're broken and can't be outtraded or killed early. You can even 1V2 if you manage to use Dominus and your Fury Bar properly. Remember you gain additional fury when low, so don't be afraid to trade.

A good combo from Renekton should be Slice and Dice -> Ruthless Predator -> 1 autoattack -> Cull the Meek -> Slice and Dice

Try to roam a lot, you deal crazy damage to midlaners.

Now that you gathered Sunfire Aegis, Ravenous Hydra or whatever core item, you should force a teamfight while you are still imbalanced. Focus the carries and don't be afraid to go alone, your goal is to deny people throughout the teamfight and even kill them.

A full combo on 1 person should kill her with Ignite. Never forget that you can chase easily with your Slice and Dice.

You are now a tank, you should probably just get nashor and deny people in teamfights.

There is not much to say, Renekton is weak as heck late game, and you should probably end the game before going Late game.