You have totally forgotten that there's just more villains out there that are better than Sephiroth or the FF series main villains.

Sephiorth from your list is the best. Why you may be asking yourself right now? I'll tell you why. Because he is an exceptionally skilled fighter. Plus his magic force is extravigantly better than any other off of your list.

Ansem was easy to beat, extremely easy to beat. Rikku, was a whimp compared to the rest on your list. Sure he looks cool, but you really need to figure out that he's just not as powerful as the rest of the main villains.

See in order to pick a good villain you gotta think of statistics not just how badd ass they are.