i thought diamond weapon was pretty fun. the hojo battles were kind of fun too.

jenova life and death battles were a little too easy, i was bored by those

and the safer sephiroth boss is fun too. especially if you used kotr on jenova synthesis, because that makes safer sephy gain 80,000 more hp. bizarro sephy gains hp too, it might be 80,000 too but i forget.
and if you use kotr on bizzaro sephy too, safer sephy gains even more hp! so it makes the last battle harder, and therefore more fun lol.

emerald and ruby weapons were sweeeeet.