I've been playing Renekton since the day he came out and I haven't seen any guide that really builds like mine. My goal throughout the game is to maximize the potential of my Ruthless Predator (RP). I would also like to point out that I mainly play 3s, so this guild is not necessarily relevant to you. This build is probably not as viable in 5s, but all the points I make should still be able to apply to any setting.

Pros:
Amazingly fun champion - This is my first AD champion (besides the occasional free champ), and I really enjoy his play style. I think because he relies so much on his abilities to deal damage he feels like a caster but with more sustainability (auto-attacks and such).
No resource - Allows for longer laning.
Nice, powerful stun - When furious, RP can do some serious damage and stun for 1.5 seconds. Sometimes that's all that you need.
Minor healing - More laning capabilities.
Dashing - SnD comes in handy so much. Escaping ganks/battles, chasing enemies, going through walls. Using this correctly can easily decide the game. Try to get the double dash as often as possible.
Interior Crocodile Alligator - nuff sed

Cons:
Kind of frail - Especially because the Doran's Shield is no longer in this build, the ability to survive is reduced. Luckily the base stats of Renekton were also buffed in the patch, but do not play like you are a tank.
Minor healing - It really is minor, so don't trust it too much. It's kind of like the 3% life steal from a Doran's Blade: not enough to sustain you in a battle or replenish you immediately, but to slowly get you back to a reasonable amount of HP
Chevrolet Movie Theater - where is it?

Armor Penetration Marks - AD champion means ArP runes. Standard stuff.

Dodge Seals - Personal preference. Dodge is one of the few things that has its coefficients directly translated to damage reduction without any way of it being mitigated (besides Sword of the Divine and some other possible scenarios). Dodging minions, other AD champs and the occasional auto-attack from a caster can really save you sometimes, especially with Nimbleness. These can be replace with other runes that either improve your survivability in other ways or increase damage output more directly.

Cooldown Reduction Glyphs - 6% CDR just from runes and then another 3% from masteries allows your abilities to be used extremely often. This also allows you to worry more about damage output items rather than CDR items. If you choose to replace these Glyphs with something else, I suggest building Youmuu's earlier because of the loss of CDR (explained more later).

Armor Penetration Quintessences - Also fairly standard. I do not actually have 3 ArP quints yet. I use 1 ArP one and 2 Fort (flat health) Quints. I do believe that having 3 ArP quints to the most superior build when compared to HP, but that is only speculation and has not actually been tested.

Exhaust - Getting your early game kills becomes a lot easier with Exhaust. Pay attention to your teammates though: if they have it too, you want to coordinate the usage of the ability for maximum up time. This also is great for getting away or chasing later in the game as well.

Ignite - Same as before. This spell combined with Exhaust gets me first blood when I solo almost every time. Nice true damage and reduced healing. This spell's damage isn't as useful late game, but against characters like Mundo, Nidalee and others it is invaluable.

Ghost - I don't usually use Ghost on any of my characters, but it has its place on Renekton if it is chosen. With this choice, it might be a good idea to go into Utility for improved Ghost and replacing the dodge runes with something else.

Go with the spell you find to be most useful (as long as it's not Clarity), even ones not listed here. These are the spells I'm used to and I use them on most of my characters.

I go all the way down the Attack Tree, grabbing improved Exhaust, cooldown reduction (CDR) and some nice armor penetration (ArP) and then going into the Defense tree for Nimbleness. It is also viable to go into Utility to get improved experience while using armor, magic resist (MR), etc. seals. However, I find the extra survivability to help immensely and the 10% increase to speed can get you some kills or save you.

In the first tier of the Defense tree I get 3 points in Armor and 1 point in MR. I find the armor more helpful because physical damage is a lost more prominent early game, which is extremely important, and minions whack you a lot. The MR can be taken, but that's the reason I choose armor.

First thing I get is Ruthless Predator. This is mainly for ganking in 3s but also has amazing damage and a short stun for harassing early game. When I choose not to gank or if I do a 5s, I usually get Cull the Meek (CtM) at level 1 and wait until level 3 to get RP. Slice and Dice (SnD) is very important and should definitely be learned at level 2.

You are able to harass right away, but once you learn SnD you have that ability increased dramatically. As seen in several videos online, the ability to Slice in, CtM (preferably first to get your Fury up above 50), RP and then Dice away can be a serious damper on your opponent's HP. Save up enough Fury to be able to have a Furious RP after a CtM, but this is not necessary. Harassing is harassing, and as long as you get damage out you should be fine. This sequence of abilities allows for the least amount of damage to be taken by you and does the most damage to your enemy. If used in conjunction with your summoner spells, this could easily go from hard harassing to a kill.

At level 4 I rank up RP again and then at level 5 I get CtM. Level 6 I get Dominus. An amazing ability that allows you to stay alive, generate Fury at an incredible rate and deal some extra aoe magic damge too. A nice thing about Dominus is that the total health given is actually granted and not healed. This means that if you are ignite (or have reduced healing from some other source), you still get the first 300/450/600 HP. Early game Dominus should be saved for when you're low on HP and want to get a surprise kill on your opponent. The extra health and damage can usually give you the edge you need to grab that kill. Later on, when you start team fighting, this ability should be saved for battles. Its CD is short enough, especially with adequate CDR, that it should be up whenever there is a team fight, but there is nothing wrong with fighting with Dominus 1v1 or full fledged fighting without it.

From here the skilling is simple. Max out RP first, followed by CtM, and then SnD, all while ranking up Dominus whenever possible. RP is done first because it is not only your highest damaging move, but needs its cooldown reduced the most. With the new change, CtM has a set CD and doesn't need to be ranked up for that reason any longer. SnD is great, but its damage output and Furious empowerment ability is so much in the shadow of RP and CtM that you shouldn't mind neglecting all but one rank until the end.

Boots - With the latest change to Doran's items, I have switched from getting Doran's Shield to getting Boots of Speed with potions. This helps with landing those early game RPs, allows you to heal up with the pots and helps you build your CDR boots earlier. The effectiveness of the Doran's shield dwindles out fairly early in the game and getting boots first eliminates that. I get Ionian Boots of Lucidity because I find that Renekton relies on his abilities far more than his auto-attacks. There are no other boots that can help you as much as the CDR boots; however, the option to get Mercury Treads or dodge boots for more survivability is always available.

Madred's Razor - First big item is Madred's Bloodrazor. The on-hit ability for Madred's is incredibly with RP (12% of opponent's hp as magic damage with a furious RP) and the extra damage, armor and attack speed all make this the item I find to be the most important. I get this first because its components are most useful early game. The 1kg piece is built first to help with jungling ( Red Buff is amazing for RP ) and to help with minions in the lane. Faster you farm creeps the faster you level.

Brutalizer - I get this to get my CDR to an acceptable level. This is something that isn't as important since the recent change to the cooldown on Cull the Meek. I haven't tried getting a Frozen Mallet before Brute since then, but I still thing the ArP, AD and CDR are so important that this will still be higher priority. Leave this alone for awhile because Youmuu's isn't much of an improvement from Brute in my opinion.

Frozen Mallet - Great item for Renekton. He needs the survivability because he begins to get frail later in the game (especially when his ult is on cooldown) and this really helps. The slow is essential for making sure that your opponent doesn't get away or out damages you (other auto-attack champs). Enough said.

Black Cleaver - With a furious RP, all of your stacks of Black Cleaver are applied immediately. The extra attack speed, damage and ArP make this item amazing for Renekton. I choose to build this after Frozen Mallet because Renekton lacks the survivability, but this is not necessarily better. There have been times where I realized that a Black Cleaver is better (mainly when I have 1850+ gold before going back and I'm not dying too much). I believe that the more immediately satisfied item building is better than saving up for a BF sword right away. Get the BF Sword before the dagger and you'll do stupid amounts of damage.

Youmuu's - Finish this off here. However, it is not a bad idea to get this before Black Cleaver because the movement and attack speed boosts can help with securing kills or downing buildings. This item is pretty much only bought because of the ArP and nice AD; the other stuff is obviously good, just not as appealing.

Other Stuff - To be honest, I don't know what to get for a last item. I put the Guardian Angel because the survivability is very important on a nuke like this. This could be replaced with a Trinity Force (which I have yet to try), Bloodrazor (life steal) or perhaps an Infinity Edge (some crit). By the time I get to this point in the game I've already won, so I don't know what the last item needs to be to be the most optimal.

Once I get my Madred's Razor, I try and get Red Buff. The on-hit effect really helps in taking down the lizard, but don't hesitate to ask for help. He's still tough and you don't want to be too low on HP after killing him (remember, you probably don't have improved neutral buffs, so you don't want to waste time going back to heal). On 3s I like to get the speed boost and sometimes the CDR/AS boost (not as good because your CDR should be fairly high already and most of your damage comes from abilities anyway). In 5s blue buff is useless on you. The CDR is unnecessary and you have no resource that Blue helps you with. It's nice to get some jungle creeps killed for the gold and experience as well, but it shouldn't be your focus. Get your buffs and get back in the lane or gank. That's my opinion.

I like to dash in an out of bushes in my lane. I stay within the bush until a minion is able to last hit, then I go get the gold and get back in. I take little unnecessary damage from creeps and enemy champions this way.

When I get RP in 3s, I am able to stun and run away before taking damage against most other melee champs, but there are factors that can prevent you from doing this. Once you begin to acquire more abilities, apply the method of harassment described above in the second paragraph of the Skill Sequence section. In addition to that strategy, stay in the bush until your opponent is close enough to be Sliced. Then do your combo. Ranged champs can't touch you in the bush and melee will be easy because they have to be in the lane with the minions anyway. Avoid skill shots, but it's not the end of the world if you take some damage. You should have 3 potions to sustain you in addition to the minor healing you have from CtM.

Renekton is an amazing champion. When I played him day one I could tell he was in need of some buffs. I'm actually afraid that this patch could have moved him from slightly UP to slightly OP, so I can only expect a nerf soon. I was able to use this build before the changes to him, but, with everyone done to this champion, this build only became more effective. I hope you enjoy the character and game as much as I do. Feed back is welcome and appreciated. Thanks.

-LK