This is my first guide so please be honest on what you think...you can say anything i dont take offense. Please rate!

Ive played at round 1500 elo but recently hit a bad streak and am now around 1400. I know its not that good but thought id let you know.

Sorry if its a long read, just want to get my points across.

IMO Renekton is an off-tank that is able to put out a lot of burst dps. For this reason i feel that the runes i listed are the best.

The ArP from the Quints and Marks will easily shred through anyone not running armor runes.
The Seals and glyphs just make it so that you can sustain lane much longer to farm and stay in there for that one last hit for a kill or get away with that sliver of health.

I run 21/0/9 for a bit more damage and the ArP. However i have ran this build with a 9/21/0 and 0/21/9 and they are both fine.

The items that i listed are almost always my build regardless of the team i am facing because i feel that it covers almost ever base and is balanced.

I start out with a ruby crystal (you can start with dorans shield, but that is why you have your Q to heal and armor seals). From there build that into a phage, this will give you both early damage and sustainability. After that i usually buy my boots one and rush for a sheen, but if you feel like you need more speed finish your boots 2 first. Go ahead from here and finish up your trinity force. This item is just all around amazing for renekton. The sheen proc for ruthless predator rips many champions wide open, while the zeal/phage effects make getting away almost impossible.

From here i like to start building tanky. This usually depends on who is getting fed or the real threat on the enemy team. My preference is to build Sunfire Cape as it adds to your lethality as well as your defense. However, if they are heavily ap stacked then go for a negatron cloak before.

After your defense is built up you have many items to choose from. I like ghostblade and bloodthirster or warmogs and atmas. This all depends on your role for your team. If you are surviving fine then go for the ghostblade and bloodthirster combo as this will add much needed ArP/damage/lifesteal/AS/CD Reduction. If you are dying quickly and being targeted then warmogs and atmas may be what is better.

Reign of Anger (Passive):
When Renekton is below 50% health he generates 50% more fury from all sources.

Not something that you really have to pay attention to but this may give you the edge in a close fight.

Cull the Meek (Q):
Renekton swings his blade dealing physical damage to nearby enemies and healing for 5% of the damage dealt up to a cap. Heal amount is quadrupled against champions. Each target hit grants 5 Fury.

When Renekton has 50 or more Fury he deals 50% additional damage and heals for 10% of the damage dealt, the healing cap is tripled. Consumes 50 Fury.

This is the skill maxed first by many due to the fact that it heals you. It is true that maxing this first may be able to heal you in lane to keep you there longer but the healing is capped.

Ruthless Predator (W):
Renekton’s next attack will swing twice dealing physical damage and stunning his target for 0.75 seconds. Each hit applies on-hit effects.

When Renekton has 50 or more Fury his next attack will swing three times dealing physical damage and stunning his target for 1.5 seconds. Consumes 50 Fury.

I max this skill first because IMO it is the most useful. With a sheen proc and 50 fury i have done upwards of 1/3 of a mage's health. Plus it is always nice to have a CC up every 7 or 8 seconds.

Slice and Dice (E):
Slice: Renekton dashes forward, dealing physical damage to targets along the way. If he hits a target he gains the ability to use Dice for 4 seconds.

Dice: Renekton dashes, dealing physical damage along the way.

When Renekton has 50 or more Fury Dice deals 50% additional damage and reduces the armor of targets hit for 4 seconds. Consumes 50 Fury.

Very nice for getting out from a bad situation quickly or closing in on an enemy that is hiding behind a minion wave. The only problem is that the slice and dice has a short range so it has its limits for wall jumping.

Dominus (Ultimate) (R):
(Active): Renekton empowers himself with dark energies, gaining increased size and health for 15 seconds. While empowered, he deals magic damage to nearby enemies and generates 5 Fury per second.

At lvl 6, this may be one of the most underrated ultimates. The health gain can save you from many bad situations and keep you alive in team fights. The damage is just a little extra and with all your CC they will take the full force unless they can flash away.

Possible Spells:
Flash (initiate or gtfo)
Ghost (catch up or run away)
Exhaust (CC their carry or Mage)
Ignite (Stop healz or get that last bit of damage)

I use flash for the surprise factor and feel that it gets you out of more than ghost does. Ignite over exhaust because you already have a lot of tankiness and slow from Tri-force

I Guess:
Smite (but i don't recommend jungle)