Hello there. My name is Ooth and to clarify this guide is not original. This is taken from Voyboy's idea of mass armor pen on an attack damage champion. Basically with enough armor penetration anyone playing a champion with under 180~ armor will be hit for,in theory, true damage. I've tested this numerous times and its awesome. However you lack the tankiness of other Renek builds like Warmog\Atmas etc. I prefer playing Renekton as a "get in your face" bruiser. For one you have a great escape (your slice and dice) your amazing high damage stun, and your AOE cleave. Even with full damage items your ultimate will still give you high survivability with the added guardian angel. I try and always use my flash offensively to jump on a carry and insta-gib them in fights.

I recommend the full armor pen reds for, well, just more armor pen. The yellows and blues can be changed upon matchup. I just picked standard armor\mresist. Quints I take AD for easier last hitting and damage for pre-6 harass in lane.

The standard 21-9 is what I run on almost any carry.

Summoners wrath: Get free AD after igniting, win win.

Brute force: simple reason free ad and you wouldnt take the free ap hah.

Butcher: extra damage to cs easier.

alacrity: attack speed is nice on any AD champion

sorcery: the reason I take this is because 90% of reneks damage comes from his long cool down abilties, these combined with items helps increase his damage output by immense amounts.

havoc: I'm sure I could put these else where but i feel a noticeable difference in my early game dominace with 3 points in this. extra dmg!

sunder: obvious reasons
executioner: kill low hp enemies easier

Defense tree is pretty much self explanatory, extra armor and mresist to live longer in lane, can swap points depending on matchup. Durability and veterans scars for extra hp for 1v1's and survivability.

Well this is the main reason for the guide. ALL THAT PENETRATION. Anyone, like I stated before, who has under 180~ armor you will deal IMMENSE damage to.

I take boots and 3 pots because simply movement speed is just so good. It helps you be aggression in lane, get in - last hit - get out, and avoid early ganks.

1-2 Doran blades depending on the difficulty of lane. I always at least get one.

Brutalizer is excellent for the early game CDR and boost in damage and arp. Since Renekton is a "melee caster" this boosts his DPS for lengthy skirmishes as well as lowering the cd on his escape (slice & dice).

Buy tier 2 boots depending on lane.

Last whisper is the bread and butter of this build. A whopping 40% armor penetration means you do basically 40% more damage.

Brutalizer into Ghost blade for the tiny boost in stats and the use is great for fights.

Another brutalizer for the added arp and cdr. yes this works.

Trinity for MORE DAMAGE, and great utlity. Movement, slow, burst damage.

Guardian cause youre super squishy late game with this build compared to other bruisers.

I max Q first because this is your main damaging ability. Heals you when hitting enemies\minions.

I max E second because with 50+ rage you get the added 15% armor penetration as well it makes renekton incredibly mobile when getting ganked or chasing down. You can go in your top bushes and juke till they come close, E through the ganker, E and flash away. Covering LARRRRRGE amounts of ground in seconds. Faster than Riven.

After your 3rd rank in Q and 2 dorans you can clear enemey waves with a simple E Q E.

I take flash on every champion unless I'm trolling or something. It helps you get out of sticky spots or make plays.

Ignite I use for the early-game lane dominance. Go all in on someone and the ignite is just way more damage and mitigates healing if they have any (against vlad, swain etc or summoner heals).

Engage with a E, W them, Q and E away. Makes for superb harass in lane. Not much can counter this as you jump in stun attack get out. Basically a mirror riven strategy. Free harass!

You can E through a lot of walls in summoner rift, i.e. baron, golems, dragon, blue buff, red buff, wraiths.

Sometimes I'll go mid with renek against certain APs. You can E through your own wraiths, Q them and quickly E out (near the part of wraiths with the SIGN). Increases your lead by an incredibly amount if youre already on par with them in lane CS and they dont wraiths.

Renektons ultimate is incredible in a 1v1 fight, you can almost always bait a gank for your jungler to counter gank or bait someone into turret diving.

Here is a screenshot of just a regular draft testing this build out.
edit: I will add a video and more screenshots once I get more time.