The best moment I had while playing Final Fantasy X was beating the International version. The Dark Aeons and Penance proved to be vastly more of a challenge compared to the Monster Arena, at least for my first time, even though Dark Bahamut still does not play around.
Sadly though, Omega Weapon was still the saddest excuse for a superboss I have ever seen.
Funniest.... I still remember unintentionally Zanmato'ing Penance when I summoned Yojimbo. I restarted the game to beat him fair and square, but I couldn't help but laugh of the strongest of superbosses going down in an instant like that.
The worst moment I have ever had in this game (and probably my all-time worst experience with any Final Fantasy) is training for two hours w/o saving in the Omega Ruins, before encountering the dreaded Great Malboro which made me ragequit. He's still my nemesis to this day in FFX and not even Penance, Nemesis or the Dark Aeons can compare to the dread this monster put me through. To this day, I get scared whenever I go into the Omega Ruins first time, because by then I most likely don't have the supplies to make Berserk or Confuse Proof. And yes, I'm aware the two hours without saving was probably the biggest mistake/fail/whatever on my part.
The real teaser that made me ragequit Final Fantasy X for a month? As soon as I reload my save, the FIRST enemy I encounter is A GREAT MALBORO! /rant