Meteo always and forever. It was my first FF ultimate and will alway be my favorite (and I don't care if it was because of character limitations in the SNES port, it will always be "Meteo" to me). Ultima is just too boring. The ultimate nature is right in the name, which makes it a little too on the nose without any clear actual phenomena associated with it and the effect is typically pretty lame looking. Nicely, even when Ultima is considered more "ultimate," Meteor is often the better spell in later entries due to the multi-hit nature overcoming damage caps. Plus, Meteo, as far as I know, is the only ultimate spell that really features prominently in the plot of a game. Summons often feature prominently, but I don't remember any other spell being treated with such significance in the plot of a game as Meteo is in FFIV (and arguably, also FFVII - arguably even more so in FFVII, but then I would have to concede Holy is treated with equal significance).