I don't mind Bahamut, I just don't like the "best because does the most damage" rationale. For the porposes of this question, I'll assume all summons do 12 damage. 12 being an arbitrary choice.

Jinn (Ifrit) from IV is pretty cool actually. But for my favorite, I'm going to have to go with Odin, in general. Just a cool summon.

Or Hades... if anyone's ever played Planescape: Torment, and cast the spell "Torment", the sequence looks amost exactly like Hades's. Strange...