Choco: VII (Fat chocobo)
Ramuh: IX
Shiva: X
Ifrit: VIII
Titan: VII
Leviathan: Tie between VIII and IV
Odin: VII CC
Bahamut: VIII

Non classic: Cactuar from VIII

One Time: Cyclops from Tactics

Ultimate: Tie between Eden and Ark

System: VIII I love boosting, and the idea of your GFs dying, however the losing memory part is dumb.