In MY opinion Renekton is easily the best top lane player. You don't see a lot of renekton but I'm telling you he is a beast, literally. High mobility, High damage, High sustain with his q, natural tank without even needing tank items, easy and incredibly fast wave clear after your tiamat. Very underestimated champion that can dominate any opponent if played right.

start long sword and 3x health pots (the long sword will build into your first item, if you chose to do so dorans shield is also viable)
first back grab pick-axe and more pots if needed
as soon as you can rush tiamat, this is such an important item on renekton i'll explain in the next section
after tiamat is built start building black cleaver, don't worry about finishing tiamat into ravenous or titanic hydra yet
after black cleaver and boots (grab merc treads against CC or just grab ninja tabi for a standard) then build titanic or ravenous hydra. This is a personal preference, try both, i personally like ravenous hydra more
then after that is when its pretty up to the situation, but the most standard core final build that i take 90% of games is ravenous hydra (titanic situational), black cleaver, ninja tab,i spirit visage or sunfire, maw, GA or randuins omen

-renekton doesn't use mana, he uses fury, which makes it easy to stay in lane without worrying about mana pots or saving spells.
-when renekton uses an ability or auto attack he gains rage, but after half rage only auto attack until you're ready to trade with a champion or heal
-when renektons rage bar is red his next ability is empowered, this is what makes or breaks renekton players
-your q heals you for HOW MUCH DAMAGE IS DEALT so this is your bread and butter. Wait till rage bar is red, then q, but try to hit AS MANY TARGETS as possible. the more targets you hit, the more total damage you deal, and therefore the more you heal. always try to hit multiple targets
-use all your abilities to fill your fury bar, spam them, but when its red only auto attack to fill it more and save your fury for a champion but keep in mind that it falls off after a while, so don't save it if you're roaming. If you're roaming and you're not full health hit stuff with your q as you run past. you don't need to finish a camp, or even have an empowered q for it to heal. if im roaming toward mid i always kill or at least q the scuttle on my way
- when you use w you stun, you also 'stun' yourself because you stand there and do a swing animation, your tiamat cancels this animation allowing you to escape easier or run ahead of them to secure the kill.
-your e can be used twice in succession only if the first dash actually hits a target, and it can be used to jump walls.
-your basic combo is e through a minion to enemy champ, stun with w, use tiamat, q for damage and heal, auto attack once and e out again
-your basic wave clear combo is e>q>tiamat (and if any minions are alive still e again) and use w on canon minions to help build fury faster
-your ultimate is hard to know WHEN to use. since it gives you a heal on-use people like to use it close to death, personally i suggest activate it before a fight or at least before you hit half heath. your ult does AOE damage so a lot of renektons take sunfire cape, up to you, i usually prefer spirit visage for magic resist and better q heal. while his ult is active he builds fury, so always try to save your q until the fury is full. if you're doing an all-in your combo should be R>E>W>tiamat>Q>AUTO>E, since your q is so late after you popped ult, plus you build fury as you attack, the q should be empowered and deal massive burst damage and heal you for a lot.
- e shreds armor, use it to blast through tanks, reducing their armor for your team to kill easier, and e again to reach the back line. as renekton you should be the chaos of the fight, using you mobility to reach the adcs and casters in the back, and using your tankiness and sustain to help fight the tanks or run after the back line is dead.
-activating your ult intimidates enemies. You can pop it just to scare a pursuer into stop chasing you or an ally, or to disengage a teamfight before it even starts if you are outnumbered but defending an objective
-since your ult does aoe and builds fury its good when taking dragon or baron, but i suggest just save it
-if you turret dive remember your ult gives you immediate health, it can save your life from that last turret shot

-renekton clears minions super fast after tiamat. if you shove your lane under enemy turret go kill the scuttle. Scuttle provides good vision and is mostly armor which you shred with e so you can kill him super fast and get back to lane
-always ward, vision OP
-tell your lanes to keep wards up. Renekton is a massive presence in a fight, so if you see the enemy team overextended ping and tp behind them. with your mobility and stun you should be able to secure AT LEAST one kill with well timed tp.
-if you push minions past the river they will always shove back, so i never shove past the river unless i'm trying to take the turret
-if enemy top lane is dead and you're under turret, attack minions fist, turret second. This is free gold from farm and your minions will still hurt the turret, plus they will live longer because they will only take turret agro one at a time. if no minions are around u hit the turret also

Renekton should be used as an aggressive top lane fighter. I definitely recommend for solo-Q play. He out-sustains most typical top lane champions and can trade with anyone in the game. Top lane is where renekton belongs, mid lane is still viable and he does well in the jungle as well if you use fury right. Renekton support... maybe if you want a high pressure kill lane but i suggest veigar if you want that xD

If you come out of laning phase behind the other top lane i suggest taking a more tank build than damage focused, swapping ravenous hydra for titanic and swapping sunfire cape for maw.

Renekton in a team fight should use his e (slice and dice) to shred front line armor and reach the back lines where all his high damage AOE from his ult, Q, and tiamat can burst down the adc or casters, and then fight or flight depending how well that went.

People who say Renekton falls off late game just don't focus right. Penetrate the back lines and multi/penta kills are easy.http://imgur.com/jRPkhNZ