Hey, this build is my way of playing Renekton. It probably isn't the best way to play him, iv'e seen a lot of good guides for Renekton on mobafire but this is how I play him :)

The reason I enjoy this build is because it means while I hit as hard as a truck riding another truck, I can take as much damage as the same duo of trucks riding another duo of trucks.
That is 4 trucks. At 6 tonnes a truck, that is 24 tonnes.

90% of the reasoning for this guide is so I don't forget my own build >.<

Desolation Marks
All of your skills do physical damage, also your going to need to hit stuff with your normal autoattack. Armour pen is kind of a godsend in my opinion for a pure physical damage hero.

Resilience Seals
Because early armor means higher % reduction meaning you can be a bit more aggressive in the early laning phase.

Warding Glyphs
Because early armor means higher % reduction meaning you can be a bit more aggressive in the early laning phase.

Desolation/Resilience/Warding Quintessences
The extra 4.5 armor/m-armor and 3.33 armor pen is just for fun ^^

I put 21 points into defense for more defense for early harassing and put the rest into offense for the point in exhaust and c/d reduction.

I recommend flash and exhaust. The amount of times I have recently killed someone by flashing next to them as the run and cut them down is probably countable on two hands.

Recommended

Flash because everyone fears teleporting crocodiles

Exhaust because no one wants to be slow next to a teleporting crocodile

Other useful ones

Ghost is useful for chasing, but the pure surprise of flash is priceless

Heal is useful for those close call situations, but if your enemies aren't dead already your doing something wrong

Ignite Useful for sniping those heroes that are just out of reach, but flash does the same job better

Why would you do this to your crocodile

Revive you should not be dying too much with the combination of your ulti/dash/flash

Clarity You'd have to be some kind of idiot

I like to get one in Slice and Dice first just to evade early 3-man ganks. Safety first yo.
Then i go into the bash once so you can dash in, bash and dash out with the two skills. "Hey why dont you get cull so you can aoe?" That's so you dont push the lane too early and kill all of their creeps.
Then you max Cull the Meek for the pushing/team fight/lifesteal-ness. While picking up Ulti and then go into Ruthless Predator for the incredible damage that it can end up doing with 50+ fury.

TL:DR
-Get one in each
-Max Cull
-Max Bash
-Get Ulti everytime you can
-Max Slice

I like to start with boots for good mobility and health potions so when your offensive you have some way to regen the health that cull did not recover.

I like getting Phage and The Brutalizer early for the health boost and the offensive-ness of both items. They also both turn into the end/mid game items that I enjoy getting on Renekton.

The combination of your ulti and phage should put your health higher than anyone else's early game, this is when I normally get Black Cleaver for the bonus damage/armour pen/attack speed. Everything that Renekton needs to be offensive, and the -45 armour pen is incredible early game.

Mid game when they start to hit harder I like to next turn your Phage into Frozen Mallet for the extra 500 odd health and the perma-slow from the attack modifier.

Next I turn The Brutalizer into Youmuu's Ghostblade for the cooldown reduction and the active which makes you hit fast and hard.

This is where I judge the game. If I am losing health quick, Warmog's Armor. The bonus 1400 odd hp is a godsend for any fighter. If the games going reasonably well though, I get Infinity Edge. Because it boosts your crit chance to 40% with Youmuu's Ghostblade which means you crit like a bomb.

I then pick up the item I didn't get when I decided between Warmog's Armor or Infinity Edge

You can do a metric **** ton of damage. For free. You have a bash, a AoE, and a good dash in dash out move. Meaning if your fast, you can:

-Slice and Dice in
-Cull the Meek
-Stun with Ruthless Predator
/If you think you can kill them [think your ally will help you]
-Slice and Dice through them
-Exhaust
-Repeat
/Else
-Slice and Dice out

Recommended to start a health pot ticking before a fight. The bonus health regen can really tip you over into unkillable.

Its important to harass them a lot while making sure you hit as many units as you can with your Cull the Meek.

At this point you should be able to hit reasonably hard and be able to soak quite a lot of damage. Hopefully you will have a more decent initiator than yourself but if you want to dash in and try and ulti/cull the meek the health out of them then go ahead. With your ulti increasing your Cull the Meek area of effect it is always nice to try and hit as many as you can with it.

Important Note: While spamming with Renekton is good, IF they have a hero with a devastating channeling ulti such as Nunu and Fiddlesticks, save your Ruthless Predator's stun for when they begin to cast it. Fiddle's feet with glow blue when he's casting Crowstorm and Nunu's ulti is a giant circle of Icey stuff. Not pretty to get caught in. Being able to stun the ulti out of these two will almost certainly make team fights easier.

Pro's
-Cooldown based so you don't have to worry about mana
-Ulti gives you good survivability
-Great damage/harass/laning
-Has an awesome shift+1
-His name is bad ***

Con's
-Not a strong initiator
-Kind of inbetween tank and damage dealer so not specialist
-Pure physical so can be countered with armor

I will never say that this is the best way to play him. I'll only say that this is how I play him. Others will get different items/runes/masteries and that's to suit their play style. I just like being able to both deal damage and take it, and I find that these are my favourite items for it.