Yu Yevon.
As this question is kind of vague, I'll answer the two I can think of this question meaning:1. The first boss in any final fantasy game is Garland, who you fight at the very beginning of Final Fantasy 1.2. The first final boss in any final fantasy is Chaos, whom you have to destroy at the very end of the first FF.
you fight it
yes he is the final boss you have to face.
Ozma is the hardest boss in the game
Nemesis
Which FF? xD In X-2, the hardest is that -bleeeep- on the last floor of the Via Infinito or whatever it's called. I haven't played the game in years, but I remember that boss was a -bleeeep-. I don't think I ever beat it...
The name of the last boss is Zeromus, an entity created out of Zemus' hate.
well the name is quite hard to pronounce but its Mike Oxbig, he is the final boss! Enjoy
Marilith or Kary (name depends on version)
you have to use tidus to talk to him and then hit him with aeon blasts
You can either fight Omega in the game (He's the final boss) or look it up on youtube. the song's called cannonball and it's by the Mythos.