Either Garland or the Mist Dragon for most of the reasons already stated.

Never cared for Guard Scorpion seeing how he was a blatant rip-off of the Whelk fight from VI. the fights from II, III, and V are forgettable. Ifrit is kind of a joke of a fight even with the timer. IX's first proper boss is actually the Rafflesia boss which was pretty clever but the auto-Trance for the fight made it a cinch, X-XIII all have forgettable first bosses.

So I guess what I'm saying is that I prefer having my first boss actually have some connection to the plot and story.