Renekton is a very difficult champion to build in a well-rounded f***ion. I find that this build works best for me when playing him as a tanky DPS. Read this build with an open mind and you might just use it yourself. Please comment, vote, and enjoy!

Pros:
-Can be very tanky
-Dishes out lots of damage
-Has selfheals for staying power
-Does lots of damage in AoE
-Has a strong lane presence
-Great solo laner
-His ability to jungle is potentially useful
-Scales well into late game
-Hard to gank (Each of his skills has a strong use for escaping)
-Great chaser
-Can leap over walls

Cons:
-Difficult to itemize
-Constantly a target in teamfights and ganks
-Relies on active rather than passive abilities to survive, meaning CC shuts him down even more than other champions

In this build you are a tanky DPS. Your job throughout the game is to get in people's faces and cause problems through stuns and damage. To achieve this you tank yourself out and aim for maximum damage output while doing so.

Early Game - You are a great farmer and harasser. Your ability to dash in, stun somebody, deal a burst of AoE damage and then dash out without a scratch on yourself is priceless for creep killing and harassing enemy champions, use this to your advantage. Your tankiness and high damage / mobility / CC allows for a lot of early game kills if played right.

Mid Game - At this point you should have already built most, if not all, of your smaller items (Ending at Spirit Visage) and are going to be a huge threat from here through. Don't waste this time farming a solo lane, run around the map and score some ganks, snagging creep farm on your way through lanes and the jungle.

Late Game - This is the time of teamfights and team pushing. Your job is to lead your team with your ambitious capabilities. Jump into a team fight and make havoc with Dominus and Cull the Meek. Your main targets for burst damage are going to be the squishies in the back, you can reach them easily with your Slice and Dice and keep them locked down with your stun. You are a great tower pusher with your high AD and AoE damage, make good use of this and your team will surely win.

The trickiest part about building Renekton is his item sequence and what item you choose to use. I discovered that the easiest way to be powerful during every stage of the game is by splitting items. The smaller items that are purchased early in the game shape you into a tanky monster, giving you all of the health, armor, MR, anti-CC, CDR, damage, and armor penetration you need to get the job done.

In my opinion, if you're going to build Renekton with tank items, they should be more focused on armor / MR rather than health. Since Renekton already has such a nice base health pool and health gains, and his ultimate provides him with health, mitigating damage is going to be needed more.

Doran's Shield - This is a great starting item, giving you health and armor which make you hard to bring down from level one, and regen to keep you in lane longer.

Mercury's Treads - Most people disagree with this choice of boots, and would argue that Ionian Boots of Lucidity is a better choice, but in my build Spirit Visage takes its place for CDR. With this build you still end up with 38 out of the 40% possible CDR you can obtain.

Heart of Gold - This item will finish your early tankiness in combination with the CDR and MR from your Mercury's Treads and the health / armor / regen from your Doran's Shield.

The Brutalizer - Once you get your Brutalizer the fun begins. This is your first and most important damage item, granting you AD, armor pen, and CDR, all of the stats that you need to begin your campaign of havoc. Some may ask why I purchase this so late, but the truth is Renekton is quite squishy early game for his role, and so your early tank items are more important and come before your early damage which you already have from the base ability damage output.

Spirit Visage - This item will finish off your tankiness for the majority of the game and contribute to your already rising CDR and healing capabilities. It's cheap and worthwhile.

Black Cleaver - Black Cleaver is hands down my favorite item on Renekton. It gives you a large boost in AD, AS, and armor penetration. Best of all, the stacking armor pen effect scales very well with Renekton's Ruthless Preditor, as each attack with it gives your target one stack, meaning with an infuriated Ruthless Preditor you can apply 3 stacks with one move! An instant 45 armor pen!

Youmuu's Ghostblade - The activation effect on Youmuu's Ghostblade is very useful for escaping / chasing / dishing out lots of damage and the bonus to AD / armor pen / crit / CDR at such a cheap price, in addition, is pretty nice.

Randuin's Omen - Remember that Heart of Gold? Well at this point in the game those pesky physical carries are more annoying than ever, build one of these and the boost to your armor will make all of your problems go away! The slow activation is a nice combo with Youmuu's for the ultimate speed and damage advantage, and as an AoE can be very valuable in teamfights.

Bloodthirster - Last, but not least, the heavy damage lifesteal item. Bloodthirster grants you extremely high damage late game, and in combination with your tankiness, the selfheals will make you unkillable!

At level one I grab Slice and Dice because it grants the most level one damage and a way to escape or chase to grab / prevent a first blood. After that I get Ruthless Preditor which proves to be the best combo with Slice and Dice in lane, jumping in to stun and auto attack, then jumping out or following it up with a kill. After this I max Cull the Meek.

I find that Cull the Meek is the most important laning tool Renekton has. It provides his staying power through selfhealing and a great amount of damage in AoE which is nice for harassing and farming. Not only this, but it is also the most base damage reliant ability he has. Meaning that if you max Ruthless Preditor first like many do, the damage gain is helpful but without it the spell still is powerful because most of the damage comes from AD scaling either way. While for Cull the Meek, most of the damage and healing effects come from leveling the ability.

I follow Cull the Meek with Slice and Dice for the same reason. Slice and Dice scales nicely with levels in terms of damage and cool down reduction. You can split the cooldown in half and quadrupal the damage by maxing this, while for Ruthless Preditor the damage gain is negligible and the CDR isn't quite as large. I also prefer a shorter cooldown on Slice and Dice because it is your main source of mobility during any fight.

Exhaust - I prefer this because it's very useful in chasing or escaping, 1v1 battles, and shutting down carries.

Ghost - This is nice for similar reasons to exhaust. Another too for chasing / escaping or quickly making it to a teamfight in progress.

There are many other options that could be explored on Renekton, but I am not going to go over them as I don't use them and couldn't explain reasoning for doing something that I don't agree with.

I go into offensive tree for the many damage benefits. I like to get the 6 points in defense for the bonus armor / magic resist which is very helpful when you are at such low values early game. The point in Ghost was prefernce and the one in regen was for lack of a better placement. Some would argue that they are better placed in the critical damage talent in the offensive tree but I find that since the crit you have is minimal and you don't get it until you buy your Youmuu's late in the game.

These are some common tanky DPS runes.

Greater Mark of Desolation - Armor penetration is a huge source of damage for any physical damage dealer. These are especially effective early game when people have only 15 or so armor and you can penetrate it all and for maximum effect.

Greater Seal of Armor - Armor is a great way to make yourself that much more tanky. I am all about early game dominance and these runes are highly beneficial when you are lacking armor early in the game.

Greater Glyph of Magic Resist - This is simply what I use to compliment your tankiness and to add to your low MR values early in the game. Some would go with CDR runes but they are later cancled out by CDR items that put you over the 40% cap.

Greater Quintessence of Health - I get these on every champion. Health is key to early game dominance, especially when you can add 100 HP to a level 1 champion with 500 or so.