This is a Renekton build that i tried purely by chance, and fell to love instantly. This build, like Renekton generally, is fairly weak early game, but becomes a beast from mid-late. I will add runes once i work those out, but for now this is just simply for the champion.

For mastery, 21/3/6 works out well, making the most out of Renekton's strengths (by increasing his crit/AD/AS) whilst also improving his health regen and magic resist.

I found the above skill sequence nets you a lot of good kills later on through sheer burst damage.

Taking Cull the Meek first is an obvious choice for wave farming along with the healing bonus, which when used in conjunction with the health pot, should keep you in lane for a fair amount of time. Slice and Dice is then taken to assist with farming and harassing/getting easy kills with lowish risk if used correctly.

Learn to aim your Slice and Dice:
Learn to track forward of minions and enemy champs to make the most of your Slice and Dice. The only time you should ever use Slice without getting the "dice" (the second dash) is when you're using it to get away.

I take Ruthless Predator later on as i personally feel it only really comes into it's own late game where you can afford to use the CD and stick around rather than having the constant risk of having to retreat.

Use Renekton's Ulti either to escape, or in a team fight. It makes him much more powerful, and can turn fights right around. It's also on a "relatively" short CD, so dont be afraid of using it.

First of all, buy a Long Sword and a Health Pot. This Sword helps you farm waves slightly easier and harass more, whilst the Health pot keeps you in lane quicker. Another option would be to take a Doran's Shield to make you more survivable. Personally however, i prefer the boost towards the Brutaliser.

Next up is again down to personal preference. Whilst some may go for boots now, i usually go for another Long Sword or if i have the gold, straight for the Brutaliser. This again helps farm lanes and harass, which is what you're going for early-mid game. I then get the Avarice blade, so that i have 2 parts of the Youmuu's, but do NOT complete it. Next, buy boots.

I choose the boots of speed, as it again makes Renek more survivable, and also helps get in and out of combat. You could take the ability cooldown boots if you so wished, but i find that the speed really comes in a lot more useful.

After boots, build the Youmuu's and then save for 2 Zeal's. If you then have to gold, gradually upgrade each into a Phantom Dancer. By this point, you should be zooming around the map and (hopefully) up in their base. Now....beastmode on....

You should be quick as a cheetah by now, and hitting like a truck. At this point, a lot of the fights are big team fights, and you should do the following.

Start my Slicing in, popping your Ulti, and then Cull the Meek. This should take a fair chunk of the enemy's hp away. Now select one, and depending on their proximity to towers, if they're running away, and how your team is doing, decide on either:

a: Dicing out of there because the situation has become unsafe
b: Dicing through your target. If you do this, then use Ruthless Predator, hopefully securing a kill.
c: Dice through the entire pack of enemies. Use this option if there isn't a particularly viable target, but the fight isn't at a point at which you need to retreat.

In conclusion, your attack speed along with your damage and skills should make most fights a pretty short affair in your favour, whilst your movement speed removes the ability for range to kite you at all, allows you to turret dive safely with your ulti, and both get in and out of combat a lot lot quicker, whilst killing pretty much everything short of a six-stack Cho'Gath in seconds.

Hope it works out, leave any comments with suggestions or feedback! Please only vote down if you have a reason!

-Mental