For me, the order is:

Kuja
Kefka
Vayne
Exdeath
Sephiroth
Golbez
Seymour
Ultimecia

In ascending order:

Ultimecia I found to be a really weak villain. I much preferred Edea as the villain to Ultimecia, even if the latter was possessing the former. She appeared almost out of nowhere, and didn't really impress when you found out about her.

Seymour I never really took to. I think he was rather creepy and played the part of the villain reasonably well but I never really looked forward to seeing him on screen, or felt particularly compelled by his reasons for being a villain. About the best thing I can really say about him is that the boss battles with him were all good fun, and the last one had some excellent music.

Golbez is another that falls into the (SPOILER)brainwashed category. I think he's a reasonably cool villain during the game, even if all isn't as it seems. I find him to be one of those villains I don't have a particularly strong view on.

Sephiroth, I think I've always been a bit unimpressed by because I was slightly contrary. However, I do think he has his own strengths. The fact that he did have such an impression on gamers, is still slightly revered by fans and still has a certain mysticism around him I think is an indication that shows that he's got a certain something about him. I don't think he's a poor villain, but definitely one I think I rate higher on style than substance. Not least because (SPOILER)we barely see the real him in the game.

Exdeath, I've always liked but never been able to quite put my finger on why. He has some awesome music and a pretty imposing character design. I think I liked hearing about him in the past against the four original heroes, and also the set piece battles that he's involved in (e.g. Battle on the Big Bridge).

When it comes to the next three, these are the ones I'm more impressed by. I can see the criticism of Vayne that people have mentioned already, such as that he isn't visible for most of the game. That, however, is something I liked about Vayne. He's a much more subtle operator. He's behind the scenes pulling the strings and playing the master manipulator role so well. He's a much more political sort of villain, and that was something I liked. I also liked uncovering the extent of his plans and schemes, and how he committed patricide.

Kefka, well, what needs to be said? An absolutely insane piece of work. I think he's a very impressive villain, both in terms of what he does and what he is. A man who sacrificed his own sanity in the quest for magical power, and who didn't hesitate in poisoning a river to kill the inhabitants of a castle. Who felt no guilt for killing the honourable Leo, or for overthrowing his own emperor. That and he's one of the most standout memorable villains I've come across in video games.

Kuja, I've always had a soft spot for. I think he's a successful villain - he broadly achieves what he sets out to achieve. Perhaps not as successfully as Kefka, but I think he had less to work with. He schemes to bring the entire Mist Continent to war, masterminds the assaults on Cleyra and Burmecia, and unleashes the Eidolons on the world. He destroys an entire freakin' planet too, in what has to be one of the most awesome hissy fits of all time. I think he has a strong mix of intelligence, a certain amount of style, raw power and the ability to manipulate and scheme to be my favourite villain. He's a man that knows when to run and when to fight. Most importantly, I think we also get to see more of him as a person than we do the other villains. We see him at moments of reflection and weakness, as well as strength and pride. In spite of having a rather questionable dress sense, he remains my favourite FF villain.