Well, Sephiroth is one of those characters who has looks, the power to destroy anything he wants AND he can last more than 5 KotRs. (I mean BOTH of them; Bazziro + Safer) He is basically a God among men who only plans on ruinning the life of a psychotic spikey-haired bastard. He even knows where to kick Cloud what with the whole Aeris-killing thing.

In short, yes, he IS FF's greatest villian.