Look, I know a lot of you will read this and think:
"What is this guy on about? Renekton is the demonic crocodile killing machine! He doesn't need any of that tanky ********!"
Well he might not, but with it Renekton is practically immortal (respectful to the skill of his summoner) and still stacks a tonne of attack damage, granting you with plenty of kills/gold.
Renekton is my main champion and i use this build (along with other builds - depending on the opposition) all the time.
Please read on and rate this build. Feel free comment or give advice (don't troll)

Greater Mark of Desolation

For my Glyphs I have focused on CDR (Cooldown Reduction) as Renekton is a combo-ability type champ that unfortunately has great cooldown times on his abilities. A lot of people like to buy multiple items for this problem instead, including a combination of Ionian Boots of Lucidity and The Brutalizer (later on Youmuu's Ghostblade), however I believe that just wastes time and item space during game.

For my Marks I have picked both armour penetration and AD (attack damage) for great strength early and late game. Renekton is a strong champion early game and the Greater Mark of Attack Damage Marks really help dominate lanes (even first blood if desired).

Finally, Seals, health and armour, two obvious necessities in a tanky build. These stack well with items including Sunfire Aegis and Atma's Impaler.

Ahhh, not quite finally...
Anyway, Quints, health and AD. The health quint really boosts the effect of Atma's Impaler and AD allowing Renekton to still dominate as a killing machine.

Okay, I am going to be dead honest with you here and tell you that I did not put much time into selecting these masteries. In saying that however, it does not mean they are wrong or not suitable for my Renekton build. In fact, these masteries help me a lot during game. If you have a problem with these, feel free to tell me where. Just don't be a tool about it :D

Offense

- I LOVE crit. strike on Renekton and this stacks really well with the Atma's Impaler and Infinity Edge.
- For obvious reasons of enhancing the effects of Exhaust.
- Once again focusing on CDR with Renekton.
- Can't go wrong with attack speed.
- A form of armour penetration I guess, helping your Greater Mark of Desolation.
- Great for helping Renekton farm gold early and mid game (seeing as Renekton isn't the best farmer...)
- More AD
- POW! Critical Strike!
- Some more Physical and Magic Damage.

Defence

- Tanky MR (Magic Resist) against those annoying casters (especially Ryze and Annie)
- Armour for obvious tank reasons...
- Yay, dodged a move... (helps especially in team fights)

Utility

- For obvious reasons of enhancing the effects of Ghost

What I Use

- Great for chasing or escaping enemy champions
- Great for chasing or escaping enemy champions. Also great for crossing the map quickly to a team fight or gank.

Harassment/Farming:
Focusing on laning early and mid game

This can also be used as a heal through the effects of Cull the Meek.

Going in for the kill
Usually do this when you have an ally there to help or when the enemy has 50% or less health (depending on the enemy champion)
...
This skill combination also works great in team fights. It also works great through the slow passive of Phage/ Frozen Mallet, however, when using this combination, be careful of roaming/ganking champions (e.g. Rammus or Alistar) and CC (Crowd Control) effects, especially around turrets.

Chasing/Ganking
Similar to the 'killing' pattern above ^
...
Great for chasing an escaping enemy on mid to low health. Be aware of other enemies ganking when doing this and CC around turrets

Thank you for taking the time to read my Tank/DPS build for Renekton.
I find Renekton a really fun champion to play as there are so many different ways of playing him.
If there are any other sections you would like me to add, please tell me and I would gladly do it for you.
Please leave comments/advice/ideas about this build and please don't troll.
Rate if you like!
See you on Summoner's Rift,
gretschzeus.