Most powerful FF Villain...

[Rez09]

Hrmmmm, well, from what I remember . . .

FF1 - Garland is probably the weakest on the list, seeing that a solo level 1 white mage can beat him if they were an asshole and bought ruse in Cornelia.

FF2 - I have absolutely no idea how strong this guy actually is pre-hell, because I have no idea if the cyclone is powered by his magic or some form of technology. Looking at the Dreadnought, however, I suspect the latter. Going on that, and considering how much of a joke he is when you kill him, I'd say his mind is his strongest point and he's on the weaker side of things. He does have an evil empire and tornado castle at his disposal, though.

FF3 - Xande is an interesting one, as Fynn mentioned. As I recall, he doesn't just put the world to sleep under the ocean, but actually stops the passage of time as a whole as a means to escape his mortality, which is impressive feat. Then again, I'm not sure if he was responsible for that or if the Cloud was. Either way, he is still a contender for the most powerful mage-villain of the series, even being completely shafted by Noah and not getting Doga or Unne's gift. He also has that cursed mirror, which is implied to be inescapable if you don't have a cheating wizard floating across the world grabbing pure hearted people to save you, which NO ONE should ever actually have. He also has access to Meteor, which FFIV tells me is super strong. I suspect he's a fairly strong character overall.

FF4 - Um . . . I don't remember Zemus being particularly powerful. Don't get me wrong, mind controlling people on a planet from the moon is pretty impressive and all, but . . . I don't think he really demonstrates any other power pre-transformation, does he? I think of him like I think of the Emperor: his mind is his greatest power. Fairly weak. Has a sweet robot, though.

FF5 - Ex-Death seems to be getting a lot of love lately whenever I encounter discussions of FF boss power, and that makes me inclined to argue against him due to my terrible personality, but . . . I don't think I really can. He has unarguable magical power even before obtaining the power of the void, and, unlike Xande, he's also shown actively using it, and I'm fairly certain he's all but kill-proof until rather extreme forces come into play, a trait fairly exclusive to him. Easily one of the most powerful.

FF6 - Kefka's power is very hard to gauge, both because it grows until the floating continent, as NeoCracker said, and because it is hard to interpret what you do see until then. He is able to wipe out espers en masse in Thamasa, but he appears to be doing it with a variation of X-Zone, which could simply indicate the espers aren't immune to death spells or that particular spell. He takes being stabbed particularly well on the floating continent, but you have no idea WHERE Celes stabs him. He kills Leo, but I don't suspect that was meant to be communicated as a fair fight. He's often credited with destroying the world, but I still blame that as much on Shadow as him, and it is the statues responsible either way. Overall, I think Kefka's strength comes less from him and more from proper use of the tools available to him, be it exploiting magical vulnerabilities, standing in anti-magic fields, poisoning rivers, disguising himself as the Emperor, or even just bringing stacked equipment with him (hello, paladin shield). Probably a more middle of the road villain, powerwise.

FF7 - From my interpretation of Sephiroth prior to being dumped into the lifestream, I generally consider him to have a solid mind and to be the peak of potential human performance, with respectable magical abilities as well, so while I think he is likely weaker than Exdeath, and would argue Xande as well, I do think he is stronger than most other villains in the series.

FF8 - I don't remember enough about Ultimecia to make a fair assessment.

FF9 - I don't remember enough about Kuja to make a fair assessment.

FFX - I don't know how to judge this one and have no assessment to make.

FFXII - I feel Vayne falls into the same category with the Emperor and Zemus, with his head being his greatest weapon. That said, he also has arguably the most effective 'evil empire' at his disposal, and access to nethicite, which, as an advantage, is hard to evaluate, but should disrupt magic to some extent.

Sooo . . . I guess my list ends up being . . .

Exdeath - Nigh unkillable; immense magical power
Xande - Inescapable(ish), unresistible stop spell; Meteor; immense magical power
Sephiroth - Peak human to superhuman performance; intelligent; magical ability
Vayne - Intelligent; Has functional 'evil empire'; Nethicite
Kefka - Potentially immense magical power; uses tools well; tons of magicite
Zemus - Limited mindcontrol; has a cool robot.
Emperor - Intelligent; evil empire; tornado castle; magical ability
Garland - Dies to angry shepherds with sticks

If I counted Sin in the list, I'd put it on/near Exdeath level; if Yu Yevon was on the list, I have no idea where I'd put him; Cloud of Darkness would be near/above Exdeath level; from what little I remember of Ultimecia, I'd put her near/above Sephiroth level; from what little I remember of Kuja, I'd put him near/under Sephiroth level; from what little there even IS of Necron, I'd put it around/under Exdeath level.