Why have you missed off Nemesis and Penance - those are without a doubt the 2 hardest!

Well, by hard I mean they take the longest time to beat.

Dark Bahamut and the Dark Magus Sisters (all 3 at once) are probably the toughest Aeons.

Don't know why anyone voted for Dark Anima - without a doubt the easiest Dark Aeon...apart from maybe Valefor.