Sephiroth. Hands down.

Seifer is a cool villian and all, and I do prefer FFVIII, but he's a joke compared to Sephiroth. Like someone just said, Seifer is just the sorceress' puppet, while Sephiroth is the ultimate enemy, as he is hell bent on destroying the world, and almost succeeds.

Compared to Sephiroth, Seifer's just a Chicken-Wuss. (I can't believe I just used that lame insult).

Sephiroth would slash him to bits in just a few hits.