Karthus is #1 for AoE DPS.
Fiddle is #1 for AoE BURST DPS.
Annie is #1 for Utility AoE Burst
Ryze is #1 for Utility AoE Burst on Demand.
Karthus is #1 for AoE dps because of Defile + Laywaste combination. It's not limited by any significant cooldown and keeps going as long as he's got blue bar. Plus his ultimate is the best AoE in the game, no limit on range ^_^.
Fiddle scares everybody with his AoE which can devestate the other team, it's harder to use to achieve maximum potential, but generally can turn the tide of a fight. Plus with his faster bouncing crow silencing spell, it simulates an AoE spell...ish.
Annie's AoE can be pretty beast if she lands the 1.5 second stun on a bunch of people. That combo'd with an incinerate means a bunch of people just took a nice burst of damage and are stunned as well.
Ryze's AoE is much more on demand, I think with Max CDR and a good rotation he can ult almost every 25 seconds.
Anivia's AoE is useful for the slow but far from mana efficient for the damage it deals.
There's lots of heros that can deal AoE damage, but baring item contributions These are the top 4 I'd recommend.
I'm a fan of Amumu with 4 Sunfire capes, Sorcerer boots, and Void Staff/Abyssal Staff personally =D. With 240 possible damage per second from capes, plus all the magic pen from boots/staff, not to mention tantrum and his ult? Amumu can get in there and mix it up hardcore.