On physical appearance, it's Valefor and Shiva!
But for power, I vouch for Anima.
Though I don't intend to overlook the other Aeons.
Trivia: Although both Valefor and Anima are generally thought to be male, they are both revealed to be female (Anima, according to Carl Jung, is the feminine side of a male's unconscious mind). After obtaining the airship and re-visiting the chamber of the Fayth at Besaid Temple, Valefor's fayth, a little girl, appears. As for Anima, she was Seymour Guado's mother before sacrificing herself to become his Final Aeon and can be found at Baaj Temple.
*Taken from Wikipedia: Mythology of Final Fantasy X
