Welcome to my Renekton Build. I prefer to build him more tanky thah many people do, I think he really shines as a meatsheild and does a phenomenal job of causing disarray to the enemy team during team fights.

Greater Mark of Desolation

Marks: Standard melee armor penetration marks.
Seals: Dodge seals synergize well with the defensive masteries.
Glyphs: Magic Resistance glyphs for extra magic defense early game, per levels are also fine.
Quintessences: Movement speed quints are my personal favorite. Being faster than almost everyone, it's much easier to chase and get that clutch stun off. If you dont have these, flat HP are always a solid choice, or Armor Penetration for more damage.

I choose these masteries as a solid base for my early game defense. I skip putting points into Ardor because AP or AS are not very important for this Renekton build. After the defense tree, I spec for ghost and go for the experience boost.

Summoner spell wise on Renekton I like to run
For me, ghost is the number one summoner spell you can take on Renekton. It helps you chase, it provides mobility in team fights, and it can help save your tail. I take ignite as my second because it works great for 1v1's and early game kills.

Alternatives:
If you personally like going for an exhaust or your team needs it, take it. It is the best alternative to ignite in my opinion.

If you dont like being CC'd, cleanse is the spell for you. Also a good alternative to ignite.

Starting items: x3 (optional to wait 10 seconds after minions spawn for a fourth potion, you will make it in time for last hits)

Core:
As a tank, Merc Treads are definitly your boots of choice over Boots of Lucidity. Combined with Spirit Visage you've got yourself solid early game MR. Add in the Sunfire Aegis and you'll have some solid defenses along with health.

Suggested Final Items:
If the other team is very melee heavy, consider building this item.
If the other team's magic users are really shining, grab a FoN
The best DPS item for Renekton in my opinion, I usually sell spirit visage and switch this in for it if i get to the end of my build.
All around a great 6th item. Good defenses, and a great passive.
If the other team is stacking armor and you cant do enough dmg, consider this. Also consider
I build this almost every single game following the core. It's slow is amazing for focusing down/chasing enemy carries, or peeling them off your carries.

End Game feel free to switch out Spirit Visage for another defensive, or even an offensive item because I feel it's midgame luster diminishes the longer the game goes.

Renekton is a very strong 1v1 if both teams have junglers, you could take mid but I wouldn't suggest it. With the right lane partner, he's also a great 2v2er.

Great Lane Partners:
Zilean-his timebombs make for great ranged harass, and if he timebombs you it makes your harassment combo (touched on later) even more effective. Also later in the game your great initiating abilities and the power to make people focus you combined with his ult and timebombs make you a destructive duo.
Poppy- A lot of nuke with two stuns is always good in the same lane.
Sion- Double stun burst lane ftw.
Soraka- Endless healing means endless harass for you, no need to be bullied inlane.

Harrassment: Renekton has really quite easy yet effective harrassment combos:

Combo 1: At level two, a very easy combo is Slice and Dice in, a quick Cull of the Meek and then Dice out. This three-step quick combo its great for early game harassment. Mid game this is also great for farming.

Combo 2: At level four, another logical combination of skills presents itself. This time, before slicing in, activate Ruthless Predator, slice in, stun, Cull of the Meek, and dice out. This is great for harassing or initiating for later fights.

Note: Dont harass like your out for blood. Especially in a 2v2 lane, its quite easy for the enemy champions to retaliate, and the damage you deal could very well not be worth it. Also, if you get outharassed, dont feel compelled to stay in-lane. Dieing is a much worse fate than having to recall and run back.

Renekton shines in mid-game. Your core itemization works for an awesome mid-game. Your quite tanky, and yet put out a lot of damage due to your skills. Your aoe is also insasne due to Dominus, Cull of the Meek, and Slice and Dice. Grab red buff and gank up some lanes!

Renekton is a beast in teamfights. Period. Your awesome aoe combined with your ability to easily disrupt and focus their carries makes life easy for your team.

Note: As said before, Zilean is an AMAZING asset with a Renekton on your team. Since you're in the thick of everything, he can just pop timebombs on you and when you get focused down he can ult you as they waste all their cooldowns on you. Perfect combo for a perfect game. Especially relevant in 3v3's. You are a monster in 3's with zil. Period.

The Wrecknekton, if played well, he can be destruction for an enemy team. Get out there, practice, and melt some faces! If you dont practice, well...you might turn out to be one of those Rednecktons....