Its not that you have to destroy a world, but I would consider the most accomplished villain would be one who did exactly set out what he wanted to do. Not many FF villains actually "win", and although Kefka does die, he did pretty much win at the whole "destroy the planet and murder millions" plan. Most other villains either want to destroy the world, or completely own/rule the world. How many FFs has the main bad guy successfully ruled the world for a significant amount of time, or actually succeed in destroying/dismantling the world?

Garland in 1 didnt actually complete much, however neat his plan was

The Emporer in 2 did get to rule the world for a while, but that was already in place before the game started so I wouldnt even say that counts. As soon as the heroes get innvolved, hes pretty much shut down rather quick

Honestly dont remember the full story of 3, but the main bad guy wasnt even shown until the last 10 minutes of the game, so ya...

Golbez in 4 managed to wreak havoc on the world, but didnt completely rule it and was eventually shut down before he could leave any permanent scars

We've already discussed Exdeath

Then theres Kefka, we know him. Gained power of gods, destroyed the world, killed millions, left the planet permanently disfigured

Sephiroth failed to destroy the planet, the only main thing he set out to do. Hes cool, but he didnt end up accomplishing much (aside from abusing cloud and later allowing him to come to grips with himself)

Eights final boss achieved time compression and was almost immediately fixed by our heroes after they entered time compression and whooped her bad.

Then Kuja, who did well for himself but ultimately not as much as Kefka

Yu Yevon hasnt gotten much praise in this thread, and it should be noted he kept Sin going for a long time, keeping the world in a permanent state of despair and panic for a very very long time.

And Vayne killed people and didnt even finish conquering another kingdom before being taken down

Wow I'm bored for typing all that