Updated 1/7/2014, 'Robin' Patch
Atomic Green Lantern is not Blitzcrank. He is not Pudge. You do not have a long enough hook, or a fast enough hook, to use it for picks. That isn't to say it CAN'T happen, but, his hook is better used to prevent escapes, compared to making fights. Naturally durable, powerful scaling, and a brutal team fight ultimate make him a solid addition to most team comps, and in a lot of cases, worth building a comp around.
Ability Order:
W start, even for level 1 fights. As mentioned, raw hooks aren't worth it, and in a level 1 fight, you're better trying to hit multiple people with multiple W's (and with both hits, in the very pack up, auto attack heavy fights), in an attempt to get your passive stacked on someone. If someone in a level 1 fight gets 4 RAD's, they die, almost no exceptions. After W, put a point in your E. You won't make gank attempts early someone is dramatically overextended, and you're better waiting for level 3 if you feel the need to rush hook. I put a second point in W most of the time, it's incredibly for jungle clear. Put your first (and for a long while only) point in hook at level 4, and from there, W>Passive>E, ultimate whenever you can obviously. If you are more worried about durability than damage from levels 7-12, max E>Passive.
Stolen Powers:
Consume, Joker's Detonate. Joker's Detonate lets you slow people that walk near creeps, cars, or meteors, which makes for an easy hook.
Starter Items: Modded Medusa Mask rank 2, the passive is strong. You level constantly, and clear quickly, so you stay at full HP and mana. Marauder Shield 2 for the damage block passive.
Core Items:
Atomic Green Lantern is a close range AP Bruiser, and needs to build a mix of durability and offensive power. Most characters don't have a proper method for AoE application of the Cosmic Staff rank 4's DoT, but Atomic GL has multiple in his W and R, and so a rank 4 Cosmic is very powerful on him, even modded if you don't think your team will have any other 'anti-tank' methods. Mogo is also incredibly powerful, high HP (your DoT damage scales from HP), high armor, and a high damage output passive, which you will be in range to utilize. Oa Gambit gives the Power Armor that you're lacking elsewhere, another shield active at rank 4, and further flat power penetration, which, in conjunction with cosmic staff, will give you near true damage (and with the mod, actual true damage) on anyone not building power armor. After these 3 prime importance items, you have a decision to make. Your next 2 items are going to be Atlantean Royal Seal (AoE slows everywhere, and a slow on hook, HP and PD for damage) and Speedforce Battery (Blink Hooks), you need to decide which order you need them in.
Optional Items:
Last item slot will be one of the listed items, but in a lot of cases, I find Eclipso's Diamond to be better than the vast majority of the other options. Your core build is a bit lacking on Power Armor, which post-patch, Eclipso's gives. Power Damage, Power Armor, and Power Lifesteal. Power Lifesteal on AtGL's R, W, and passive is incredibly powerful.
Final Notes:
One of the strongest late game characters, while being powerful early on as well. His hook isn't quite as powerful a pick tool as the Blitzcrank and Pudge hooks it's been compared to, it's still incredibly strong. Don't focus on his hook, focus on the power of the rest of his kit, and use his hook to fully utilize the rest of your kit.