Definitely Bahamut,he is overall my favorite summon in every FF game.He is strong (one of the strongest summons) and he looks cool.To me he is even better than Neo Bahamut and Bahamut Zero from FF VII.
His appearance in FF IX is just awesome and I like that he plays a small part of the story.
I also like the video scenes of him in FF IX.Well there is nothing more to say than Bahamut rules.