Eidolons

I don't really use Garnet in battle (I think she sucks in comparison with Eiko). But even if I never use Garnet, Bahamut fans will triumph!!!!!

Bahamut
Madeen
Phoenix
Carbuncle

EDIT - I didn't like FFIX Summon System. I really like FFX and FFVIII Summon System.

Zidane: Bahamut's animation pwns them all, Ark takes way too long but it's fun to watch sometimes, battlewise Garnet using Bahamut pwns Eiko using Madeen or Holy any day IMO, because you can't soften Mega Flare, while Terra Homing, Eternal Darkness, and Holy have elements that can be soften and even be absorbed.

Leviathan and Bahamut, duh!

I like Madeen and Ramuh.