Caius, obviously. He's the only person with an interesting skillset that look like they belong in a fighting game and humanoid enough for a player character.
Bhunivelze and Barthandelus are no less monsterous then Jecht, Ultimecia and Exdeath are when they fight.
But they're not the size of them. Ultimecia never turns into her final form to scale in Dissidia. Barthendelus and Bhunivelze will look laughable scaled down, since size's the only thing they got going for them.