Going to take the liberty to extend this to general antagonistic forces as otherwise I'd have to cut out what easily takes the top 2 spots of the list. Sephiroth would still be up relatively high but Seymour would not.

1. Sin
2. Shinra/Turks
3. Kefka
4. Ultimecia
5. Cid Bunansa
6. Exdeath
7. Kuja
8. Caius
9. Shuyin
10. Golbez/Zemus
11. Barthandelus