Ultimecia is my favorite because shes the only female villain but also because I think she's possibly the strongest villain too. Everyone else might might locate themselves in space or the centre of the earth, but only Ultimecia resides in the future and plots to compress the past, presant, future and all of space into a single moment to recreate all of reality and rule as a Goddess XD Could any of the other villains possibly top that? FF8 is also my altime favorite FF because the plot and level design was so beautiful and intricate, the GF system was great and FF8 was the first (and best) to have CARD GAMES! 8D
PS: I can get 100 curagas before the Timber mission which becomes over 5000 HP tho I prefer to pop it into Vitality, increasing it to over 120.
PPS: If anyone wants, I can tell you how to aquire Lionheart on disk 1
Only villain who i can forsee rivaling Ultimecia would be Kefka because he assimilates Godly power that possibly surpasses Ultimecia and actually succeeds in defacing the earth, loved his characteristic laugh and his awesome jester look. However his evil plot was simply mindless destruction whereas Ultimecia had the entire universe and all of time on her wishlist

I think Sephiroth is really cool, love the sword and the trenchcoat and his hairstyle was my childhood crush, however, hes an angsty emo compared to a professional antagonist like Ultimecia and Kefka