This is a weird list but I thought it would be fun to discuss.
For those who don't want to read, here's the countdown. Note that XI and XIV are skipped over and replaced with two FF bosses from non-numbered games. There are obviously spoilers here as well. Starting from easiest to hardest.
15. Yu Yevon
14: Orphan
13. The Dark King (Mystic Quest)
12. Emperor Mateus
11. Safer-Sephiroth
10. Ultima The High Seraph (FFTactics)
9. Ardyn
8. Kefka
7. Neo-ExDeath
6. The Undying
5. Zeromus
4. Necron
3. Chaos
2. Ultimecia
1. Cloud of Darkness