Renekton is usually focused in almost every fight. Whether it's because he ults into an enormous Alligator (or Crocodile?) or because he is deemed "squishy" you have to focus on your defensive abilities in order to survive the early game harassment. If you play a Brusier as i do, you're super aggressive and usually initiate fights, and/or tank as many players as you can so that your team can capitalize on the opportunities. I play Renekton in the following manner so that I can dish out huge amounts of damage, cut rhrough tank armor, and chase down just about anybody.

Marks: Armor Pen - slices through every mage in the early game and cuts through most tanks. you'll be hitting players at 0-15 armor, pending their rune selection.

Seals: Armor - grants durability

Glyphs: Magic Resist - flat resist grants further durability

Quints: Move Speed - 3 quints grats move speed 3 with level 2 boots. enough said.

Offensive powerhouse + durability

Boots first - I never stay in my lane longer than 5 minutes. It's just how I play. I like to harass other lanes, search for jungelers, provide a gank potential, or even just chase down my own lane with a partner. Health pots keep you in the fight longer.

Boots of Lucidity - with the additional tenacity items, 15% CD is ******ic.

Warmongs - 900+ health after fully farmed and health over time when running out of battle, this item keeps you in the fight longer and synchronizes with Atmas for heavy damage

youmuu's rageblade - armor pen + attack speed + crit, its everything that a tank killer could need. plus you can chase people down faster as well.

guardian angel - 1 up life

cloak and dagger - tenacity baby! and more crit is sexy too

atmas impaler - further durability and critical strike; stack it up baby

As i previously stated, I'm super aggressive with Renekton. you're an armor shredding powerhouse with the ability to chase down and stun your opponents. Synch with another CC champ and you'll down anybody. Renekton attacks very slow, so speccing attack speed really builds your rage faster and gives you additional crit opportunities.