This is a tanky DPS build for Renekton that provides high survivability early, and creates a powerful mid and end game. I find that Renekton has enough damage early to be competitive in his lane without having to buy damage items. Instead, focusing on his survivability gives the ability to get that tower dive, survive ganks, and even turn them around on the gankers. After survivability has been built up, the build switches focus to damage to stay competitive with other high damage characters,

For runes take:

Marks - 9x Armor Penetration
Seals - 9x Armor
Glyphs - 9x Cooldown Reduction
Quintessences - 3x Armor Penetration

The armor penetration Marks and Quints are pretty self-explanatory, the only magic damage you deal as Renekton is your ult, which is not stressed at all in this build, since getting AP would only cripple his survivability and damage. So these will buff nearly all of the damage Renekton deals.
The armor Seals provide extra survivability early game, and take up a spot that would otherwise be a secondary rune.
The Cooldown Reduction Glyphs are incredibly powerful on Renekton. Having them will give better staying power by increasing the frequency at which he can use Cull of the Meek, as well as assist him in chasing or ganking, since his stun will be up more often. The CDR from the glyphs is eventually wasted, so it is possible to switch these out for either Magic Resist for more survivability or a physical stat such as Attack Speed. However, having the early CDR is very useful, since it takes building your Ionian Boots of Lucidity, a Spirit Visage, and a Youmuu's Ghostblade before you will go over the 40% cap.

Pretty standard melee/carry masteries, high into the Offensive tree to increase damage, and up to Greed in Utility to improve Ghost, and get the Exp boost.

Possible Changes:

Move 2 points in Offensive Mastery to Alacrity.
I take Offensive Mastery because Renekton is a great farmer, and, even though it is a small damage bonus, it will help with farming minions early game so that you can build items quickly. Also, this build does not focus on attack speed, and instead focuses on being a skill-based AD champion rather than a high crit/attack speed carry. Regardless, it is possible to move these points if you feel the AS will help more than the bit of extra farming and lane pushing.

Move 1 point in Archaic Knowledge to Alacrity
Again, like I said in the introduction, Renekton's only magic damage is his ult, which deals AoE damage. So Archaic Knowledge will help in team fights when you can ult and hit 3-5 of the enemies. However, again, if you feel your AS is more important it is possible to move this point. If you move this point you may want to move Offensive Mastery to Alacrity as well to get more out of it.

Move 1 point in Greed to Utility Mastery
Since this is a melee build, having Red Lizard as often as possible is advised (unless there is someone on your team better suited for it, for example, a carry who stresses high attack speed and will proc it more). The increased duration is nice to have, but it will cost you a bit of gold that will help you to build the survivability items quickly and get to the damage. While every build is better if it can be built quickly, this one likely scales better than most, since for the first roughly 15 minutes of the game are spent building survivability items that help your killing ability very little. So obviously, the faster you can get to the damage items, the better.

There are many options here, and the majority of them viable. It just depends on your playstyle.

I prefer Ghost and Exhaust because both are able to be used in both offensive and defensive ways. You can ghost out of a gank, or to catch up to an enemy. You can exhaust an enemy chasing you down, particularly if they decided to run under your turret, or exhaust a fleeing enemy who may otherwise have gotten away to get a kill.

Flash is also similar to Ghost and Exhaust in that respect, but Slice and Dice serves as a dash/flash already, so I prefer to take Ghost and Exhaust.

Many other choices are viable, particularly depending on your team. Taking Ignite, for example, isn't a bad idea for early game, and can be useful when facing a Mundo, Vlad, or other champion with high healing or regen. I'll leave it up to your discretion what to take, but remember to change your masteries accordingly.

Starting off with Cull of the Meek provides some healing, damage, and ability to last hit minions. Max this first to improve your damage, and your healing, the two things that let you get kills and get away with it.

Grab 1 point in Slice and Dice at level 2 to be able to harass, escape, and chase

After that, max Ruthless Predator for more damage output and your stun.

Put points in your ult whenever possible. There are dozens of ways you can use your ult. It increases your max health, so it can be used to tower dive, or to narrowly escape a gank. Using it in team fights will deal a pretty fair chunk of AoE damage to enemy champions that decide to get to close to you, and it also increases the range of your spells, including the radius of Cull of the Meek so you can hit those casters that are just outside of the fight, or people who are running away, if you're just a little too far behind them.

Start off with a Doran's Shield and a Health Potion. These items will provide you incredible staying power, rivaling some of the most stay-able champions, such as Mundo and Vlad. The health regen and armor on Doran's Shield will stop damage, and let you recover the damage that you do take. If you're careful, you shouldn't have to use the Health Potion, but better to have it than nearly die and wish you'd bought it so you wouldn't have to blue pill. Cull of the Meek should provide you most of your healing, but if you find yourself low on health, don't be afraid to pop your pot. Particularly useful when laning against a few annoying harassers.

Try to last hit minions as much as possible, if you have high health try to use Cull of the Meek to get 2 or 3 last hits at once. Obviously, if you're low on health don't hesitate to Cull of the Meek on CD, even if it won't get you a last hit. You can't last hit at all if you're dead. A great tactic for doing a bit of harassing, farming, and healing all in one is to wait until you have 50+ fury, Slice, preferably from the bushes, into the enemy minions, Cull of the Meek (which, with the extra Fury will most likely get you a last hit or two, and give you some good healing), and then Dice out. This method is not advised if you are low on health and/or the enemy champions in your lane have large amounts of CC. If low on health, just be careful, try to get your laning partner to let the enemy push up near your turret, and once under its protection Cull of the Meek as many minions as you feel comfortable hitting. Just remember that if you're trying to hit the caster minions enemy casters may be able to get a few spells off on you, so use caution and discretion.

Your goal is to, on your first blue pill, buy your Boots, Heart of Gold, and as much of Spirit Visage as possible, preferably the whole thing. This is easily possible as long as you're careful, and work on last hitting the minions as much as possible. Obviously, a kill and assist or two will help out greatly as well. Now you may be wondering, why these items?

Ionian Boots of Lucidity give you your necessary movement speed boost, while giving you Cooldown Reduction, one of Renekton's most powerful stats. This build stresses a skill-based attack damage (somewhat similar to Pantheon, in theory if not practice), so the more CDR you have the more times you can Cull of the Meek for health back, Stun the enemy, or dash after an escaping foe. Since skills are the majority of your damage, the more you can use them, the more damage you can deal, not to mention the more useful you are to your team, with stuns on dangerous characters, and a well-placed dash to stun an escaping enemy with Slice (and Dice) and Ruthless Predator. If you have CDR glyphs, eventually you will be wasting about 7% CDR that is over the 40% cap. This isn't a huge deal, however, if you don't want to, it is possible to buy Berserker's Greaves or Boots of Swiftness. However, this build doesn't focus on Attack Speed, and between your Youmuu's active, Ghost, and Slice and Dice, you have pretty good chasing and escaping ability, so the extra movement speed isn't entirely stellar, but if you have some problems with wasting CDR, feel free.

Heart of Gold helps balance out that horrendous health pool Renekton has, as well as giving extra armor, and the one thing Renekton needs most; money. Increasing your survivability while increasing the rate at which you get future items? Sounds like a pretty good deal to me.

Spirit Visage is huge for Renekton, seriously, this may be one of the greatest items for Renekton. Like Heart of Gold, it helps to bring up that low health pool, provides Magic Resist for survivability against casters, provides Cooldown Reduction, and also increases healing and regeneration by 20%. That means that the huge healing you got every time you use Cull of the Meek just got increased by 20$. This is huge for survivability, particularly in team fights where you may hit 3-5 champions and maybe a few minions or two besides.

After you get these items continue laning if you're still in the laning phase, but it should be over pretty soon if it isn't already. After it is, run around, protect towers, gank, and be sure to get Red Lizard if you're the one on your team that will put it to the best use, or if nobody else is bothering to get it.

The next item you'll go for is a Youmuu's Ghostblade, starting with the Brutalizer portion of it first, if you can't buy the full thing right out. This provides some great Cooldown Reduction, which should push you over the cap assuming you have CDR Glyphs and Ionian boots, as well as some Armor Penetration to help counter the tank items. It also provides some crit, which is always useful. The active for Youmuu's is also great for getting some extra damage in team fights with the attack speed, and escaping or chasing with the movement speed boost. It also provides some much needed attack damage that we've been lacking for awhile.

After finishing Youmuu's build a Frozen Mallet. This provides some great survivability in the form of health, some great team utility in the form of Frozen Mallet, which should proc on the majority of, if not all of, the enemy champions in a team fight thanks to Cull of the Meek, as well as providing some more attack damage to boost your abilities.

Next you will sell your Doran's shield for a Last Whisper, which will give you a good increase to your attack damage, and well as providing huge ArPen, both of which should help you to deal large amounts of damage.

If the game continues on long enough replace your Heart of Gold with a Bloodthirster for even higher damage, and Life Steal that should make up for the health and armor loss in the survivability department. There are several other items you may wish to buy instead of a Bloodthirster, depending on the situation.
You can turn your Heart of Gold into a Randuin's Omen providing a good active for team fights, as well as some more survivability.
Warmog's Armor will increase your survivability with higher health, as well as regen which is buffed by your Spirit Visage
Force of Nature for a high magic damage team, some extra movement speed can't hurt, and the health regen is again buffed by your spirit visage
Infinity's Edge will provide more crits and damage from them, as well as more flat attack damage, if you have no prolems with survivability at this point, it's a good choice.

As I find more of these I will be sure to update this page.

If there is a Karthus or Caitlyn ult, or Mord has put his ult on you, and you fear you might die from it, pop your ult if it is up. It will increase your health, hopefully saving you from death

If you are trying to escape whether from a gank or a team fight that went the wrong way, remember to Cull of the Meek whenever it's up, particularly if you're passing a minion wave. Using it doesn't slow you down at all, and gives you back some health, increasing your chances of surviving and getting away

Try to learn all of the jungle walls that you can Slice/Dice through, and remember to take advantage of the 2nd half of the ability. If you can Slice, whether through a wall or just up the path to neutral monsters, Dice will be up, allowing you to dash down another path, or through a wall, escaping a gank.

Hopefully this guide has been clear and helpful in helping you to play and be successful with Renekton. If you have any problems, would like to add some more knowledge, or didn't understand something, please feel free to comment and ask.

Also, please don't down vote without a comment. If you don't like the build, let me know what your problems with it were, and maybe I can help you, or fix something in the guide to help others.

Thanks.