Including Yu Yevon over BFA, but Safer Sephiroth over the final duel, goddamn double standard bs >.<

*shrug* I dunno. Haven't fought Chaos, Mateus, CoD or Ardyn but I think I'd put Zeromus, Necron and BFA highest with Ultimecia and Orphan following soon after (and Caius' Bahamuts too if that counts) and Neo Exdeath, Kefka, Safer Sephiroth and the Undying further down. All bosses can be made pretty easy by going out of your way even a little though.