Well, best Aeon is dependent on your opinion, but my favorites are the Magus Sisters, Anima, and Yojimbo.

Magus Sisters
I enjoy using these ladies as it's like having three Aeons at once obviously. All three have unique special attacks that does an amazing amount of damage too of course. Camisade, Razzia, and Passado just plain own, and are devastating no matter how powerful your sisters are.

Yojimbo
I like him for a variety of reasons. His entrance, whether shortened or extended, is just awesome with that music as he steps in with his twirl of the cape. Second, his Evasion make him invincible against Physical attacks, and that's always a nice plus. Third, he can do a great amount of damage regardless of the gil you pay him. I relly like the animations for them too, with Kozuka being my favorite.

Anima
My favorite Aeon of all Aeons, I enjoy using him simply because of his destructive power. His attacks are incredible, and Pain is always a fun move to use.