BahamutZERO, no question. I never get tired of seing him launch his attack from orbit. Other than that I have to say Bahamut is my favorite in every other game, though I can't say I particularly like the look of him from FF10 onwards.

He was actually the ultimate summon until FF6. In FF6 Alexander & Tritrioch have slightly higher powers (114 & 110 to 92) but Bahamut ignores defense. So I don't know how it works out. And Crusader targets everyone, so it is debatable which is better. Only FF7 & after was there a pattern of having a definitively better summon.

Overall I can't say I am particularly enamored with having a summoner in the party. I don't mind the fact everyone can't summon everything, but take FF10 for instance. In a game where the system is designed around alternating in 6 characters to your party having one that is an order of magnitude more useful is poor design. That and I hate having separate mechanics run for summons. I want to play the damn game, not a stupid summon minigame. I'm looking at you FF10+.