The last boss to be considered for FFX is actually Jecht [Braska's Final Aeon]. He can be a pain if you take on him with mediocore equipment, or he can be a fly that can be ticked away with one or two blows if you have all Celestial weapons. A friend of mine took him down with two blows.

Yu Yevon is more like an extension, much like the final battle between Cloud and Sephiroth in FFVII [easy as hell as well] where the player gets to use Omnislash even if he hasn't acquired it at the Golden Saucer.

And oh yeah, Final Fantasy X is the first (and the only) videogame that made me cry at the end.