As i've played Renekton for a long time, this is a build i really love to use.

this particular build is great for going solo top, because Renekton has great regenerating ability.

however, this build is viable for jungle Renekton, for that, scroll down.

Marks and Quints:
The reason you take this much armor penetration is for your skill's Ruthless Predator and Cull the Meek, these attacks hit very hard on lvl 1-6 and makes it very easy to gain farming and poke your enemy.

Seal's:
Armor seal's make you keep in a lane longer, because your starting runes + your mastery makes you resilent versus minion's and enemy's without any Armor Penetration.

Glyph's:
Mres Glyph's are great for extra defense early game against ap casters, combined with Mercury's Treads give a great spell resistance

24/6/0
Because there are so many points in offence, last hitting and poking enemy's is a lot easyer.
your armor penetration + your masteries make your hits hit around 5-12 dmg harder than showed, and if you start off with good damage Cull the Meek and your Vampiric Scepter make you regenerate very fast.

For the few mres and armor you get, it still is a good early game advantage versus minions and ap casters.

Why start off with Vampiric Scepter?
because then your Cull the Meek skill and your Ruthless Predator skill heal you for a good amount.

Why choose Mercury's Treads over *other boots*
Because the extra tenacity and extra mres make your escapes easyer.

Why Randuin's Omen as last item?
Because this build isn't the usual tankyton you see in games, this build is for maximum damage, Randuin's Omen is just a extra for survivability. the other reason is because then you can gather more gold from you Heart of Gold

Trinity Force? why?
Because it works perfectly with Renekton, he is a burst melee-dps, all his skills activate Trinity Force and makes your Ruthless Predator hit EXTREMELY hard mid-late game.

Wriggle's Lantern?
Yes, i think wriggles is the best item for any dps, as it gives armor, damage and life steal and a free Sight Ward every 3 minutes.

You're up against a team of 3 ap carry's, 1 support and 1 ranged dps.
that is a problem, this build is mainly focussed on damage, but being nuked from afar can give a problem or two, as the magic resist is low.

Therefor, there are two optional build's:

Regrowth Pendant-> Mercury's Treads-> Wriggle's Lantern-> Atma's Impaler-> Zeal-> Phage-> Sheen-> Trinity Force-> Negatron Cloak-> Force of Nature-> Infinity Edge-> Warmog's Armor

This build mainly focusses on health, health regeneration and damage, although you become weaker you do stand stronger against ap casters like Brand and Annie because they can nuke you, and have (for a fight's duration) long cd. your regeneration from Wriggle's Lantern and your combined health regeneration make it easyer to lane longer and stay in teamfight's. this however makes you more of a Tanky- Renekton than the burst dps- Renekton the build is mainly about.

Allright, 4 auto-attacker's and a support!
this is a very small chance to happen, but if it does follow this item build:

Boots + 3 Health Potion's-> Heart of Gold-> Ninja Tabi-> Wriggle's Lantern-> Phage-> Zeal-> Sheen-> Trinity Force-> Randuin's Omen-> Thornmail-> Infinity Edge

Auto attackers stand no chance against a Renekton with this build. exually, they dont stand a chance against any melee-burst with this build, while nullyfiying their damage you simply slow their attack speed and deal damage back. your burst damage from Trinity Force + Ruthless Predator stun an opponent and deal great damage, followed by Cull the Meek and chasing with Slice and Dice make you a dps nightmare. but make you a Tanky- Renekton

Farming:
Farming with this build is very easy, if you can last hit properly you can get 20+ minion kills at the 5 minute mark. and while laning, dont forget to use Cull the Meek close to enemy's, you regen more health and when used with 50+rage, more regen and damage are dealt, its a poking heaven.

Laning:
Easy, simply because in a solo vs. solo lane you dominate. your damage, regeneration and defence are strong enough to keep minions from harming you hard and enemy's from killing you.
the laning is easy, the only thing you do is repeativly poke your enemy with Ruthless Predator and Cull the Meek, if they're low they will either go to base and you get some free pushing, our they make one mistake and you eat them.

Skill use:
Cull the Meek is used around minions that are close to death and to your enemy, this makes you poke your enemy and farm minion's, and you regenerate health.

Ruthless Predator: is one of my favorite skills in the game, as for you can simply stun and enemy while hurting them badly, and towerdive for a kill, or just to poke alot and make them go base often.

Slice and Dice:, its a good skill. but it doesn't deal a lot of damage, the only thing you use it for is cutting edge's and chasing opponents, which is the reason we max it last.

Dominus: Its a good ultimate, but its not very strong. the best use is to escape from enemy's or chase them with it. the additional damage and health make you a hard opponent to chase, as for they get damage and you can suprise them with a Ruthless Predator and Cull the Meek combo.

Know your place.
The thing you do as renekton isn't going head in 1v5 hoping for your team to dash in too, but to poke a enemy that is close to you. the tank or major cc has to engage first. your job is to pick off the squishy, wether its a ranged dps like Ashe or a squishy jungler like Nocturne.
this ofcourse, isnt always the case.

with your team you basicly do this:

Tank/Major cc engages
you run in for the squishy
rest of team is damaging squishy/not-so squishy
tank is being ignored
entire team is dead but for tank
attack tank.
probably ace or almost ace.

this is the basic of any team fight, but dont forget that late game not all champions remain the squishy early game champion, therefore late game its the best to focus their major dps and not their squishy'est member.

Renekton however is good at rescueing teammates too, imagine:

Ashe is being followed by Nocturne and Brand
Ashe is low, Nocturne and Brand about half-hp
you run in, Slice and Dice Brand and/or Nocturne
Dominus, Exhaust Nocturne, Ruthless Predator Brand + Cull the Meek
either they are very low or dead, easy pickoff for Renekton.

Jungle Renekton? Are you mad?!
Nope. it is extremely viable.
follow these steps, they will guide you to lvl 6

This is the case, enemy has jungler, you are your team's jungler.
you have Smite + Flash and the same runes and masteries (besides the Exhaust for Smite mastery) as the lane- Renekton

Here is my video-Tutorial:
Play Video /league-of-legends/item/cloth-armor-12

I really enjoy Renekton and think he is a great character to play, even though he is under-played very bad. i hope you give this a try and give me tips on how to improve my guide, and ask me anything about the playstyle of Renekton.