Now keep in mind we're never going to truly reach a definite answer on this subject. There is no clear-cut answer, only people's opinions.
I simply want to see which side has the majority of the opinions.
I'm sticking with my belief that Jecht is the last boss. The battle with Yu Yevon is as the battle with no-shirt Sephiroth was in Final Fantasy VII. That is, not really a battle.
Nobody calls No-shirt Sephiroth the final boss of FFVII, so why do some people insist that Yu Yevon is the final boss of FFX? It isn't possible to lose to either. (Well, it is possible to get a game over on Yevon, but you have to make it happen yourself by stoning your own characters.)
I just want to see some opinions here. Who's the last boss of Final Fantasy X?