The best villian is by far Sephiroth, but my favorite is Seifer. I also liked Sin because it didn't have a plan like other villains, it just destroyed. Seeing what it did to Kilika and Operation Mi'Hen proved that he, unlike the other villians, simply killed to kill.

I ldon't really have a reason, but i have always liked Seifer. I was hella pissed when I found out you could only use him for a little while.