First build on here so bear with me. I've tried many different builds with renekton most of which were very inconsistent this is what i have found to have the most consistent results... Wins.

Pesonally I find him best done with high cdr in the early game I rush boots first this gives me the advantage of having higher speed to chase the low hp champs at start and gives me a nice 20% cool down reduction.

2nd item the brutalizer is for the same thing an inexpensive item so u can get it early game and provides some more cdr so you can push more dps seeing as he only has cdr to worry about with no mana/energy.

3rd item is the sunfire cloak this is a survivability item as well as a lil damage. The thing i like about this is the it pairs well with his ult and the damage is useful I cant count the times my ult/sunfire got the kill when my abilities were on cool down.

4th item is bloodthirster all around good choice as a melee the life steal is nice the i love this because you get lifesteal in addition to more damage which in turn increases the life steal and damage done of your culling of the meak.

5th item is black cleaver its a good choice if like me you find yourself not having enough to get those tanks down this will help you mow them down along with just normal champs.

6th option is dependent on your team comp and the other team. a good stable choice tho is guardian angel this will make you a less "tasty" choice to the opposing team because you will just come back also the little armor is nice.

Runes are pretty straight forward armor pen to bring them down fast. Dodge for a lil more get away n survivability. CDR because... come on if i gotta explain dont play him.

With the masteries its a pretty straight forward melee dps but i choose to go for the dodge just so you can get away in a sticky situation.

At around lvl 9 or 10 you can really start going with the farming between your slice'n'dice and cull of the meak you can wipe out a group of minions very fast.Slice into minions to gain the fury hit your cull for max hp steal and damage then dice out. At this point in the game u should be able to make some jungle passes which despite what your team might say blue buff is good for re. This brings you more cool down reduction which is your bread and butter.