Everything about Urgot screams AD. Get the appropriate runes. Don't make me baby you. Also, his W is a shield that increases with AP, if you're into that thing.

Armor pen and AP is not exactly the most important thing when I was considering my build, and after extensive tests in the field, pure, unadulterated Attack Damage proved the most useful when building this champion with rampant manslaughter in mind.

In game, you may want to switch the order I've provided, but in the past 10 victories I've had with Urgot, I've managed to keep a positive k/d spread with the order I've provided. It gives you the highest cooldown reduction first, so you can rain hell from the sky with your Acid Hunter missiles every second (which turns out to be a higher frequency than his regular attack speed). Afterwards, to reward yourself for all your killings, a mana upgrade is a necessary buff with Tear of the Goddess, which increases your mana by 4 every time you use an ability. Late game, these additional manas will rollover to your Manamune, so fear not! Additionally, for increased kills and farming potential, BF Sword precedes the Manamune, for maximized damage, to bring this brilliant champion to fruition.

Exhaust will keep your foes from running out of range from your Acid Hunter missiles, or can possibly be used to slow your foes down for a more precise Noxian Corrosive Charge. Ignite is for those pesky buggers that do somehow escape your range of fiery raining acid death, and compounds the damage over time in conjunction with NCC.