Only Ultimecia can be considered the best among my choices because I don't own a PS2 therefore I don't play FFX.

Sephy - No spamming on KotR and you can still beat him easy.
Kuja - Relatively easy, cool music ftw.
Necron - I hate him, cool music ftw too.
Ulty WEAPON - He's okay...... but he ain't no final boss or whatnot......

Overall......

5. Safer Sephy - OWA rocks.
4. JENOVA - Because of J-E-N-O-V-A.
3. Necron - Cool music too. Hate his Grand cross.
2. Kuja - Uber cool music. Love his Flare Star.
1. Ultimecia/Greiver first form - Super sool music and super hard, epic fight. I suck a lot so I killed her 45 minutes forst time and 30 minutes second time.

ULTY/GREIVY FTW!!!!