Which Villain Accomplished The Most?

[Firo Volondé]

Kefka became a god and ground the world under his boot heel for a year.

And Yu Yevon terrorized the world for 1000 years.

I don't consider Yu Yevon the villian of FFX. Yes Sin needed to be stopped, but it didn't actively try to destroy everything in the world.

Not all villains set out to destroy everything, nor it is a prerequisite that they do in order to become villains. If so, villains could only exist in fantasy/science fiction, which is obviously untrue.

In fact, for something so powerful, it did almost nothing in that 1000 years.

True. It only managed to send the fabulously futuristic Spira back into the Middle Ages, keep it there for 1000 years, halting any technological advancements, destroy any settlements that grew too large for its liking, and accumulated a kill count that almost nothing in the whole history of fictional stories can match.

The Yevon religion did more damage to the people of Spira than Sin by holding them back technologically, but as was said before, Yu Yevon didn't really create that religion.

Oh didn't he?

They said that the people of Zanarkand became the fayth, that they had called Sin. And that the man responsible... was none other than the summoner Yevon, ruler of Zanarkand!
...
Yet the people of Bevelle still feared Yu Yevon. It was to quell his wrath that they revered him, and first spread his teachings. And so were born the temples of Yevon. I suppose it's possible Yunalesca had planned it that way from the start!

The damage Sin cause never seemed to stop the people of Spira from quickly rebuilding, or creating fairly massive cities like Bevelle or Luca.

Sin was still following the programming set down for it by Yevon; it did not seek to crush Spira entirely, but to prevent it from getting too advanced. If Sin hadn't let them rebuild, the people would have lost hope; something both Yunalesca and Mika considered catastrophic.

As for the cities;

Tidus: What about Luca? It's safe here?
Yuna: It's not any different, but the stadium is here. The Crusaders fight to protect it with all their strength.

In any case, Sin wouldn't have destroyed Luca, as that would've eliminated blitzball and with no entertainment, the people would lose hope. And not only does Bevelle have the protection of the warrior monks, but has a temple that summoners need to pray at to get the Final Summoning. No Final Summoning, no hope.

Sin (and by extension Yu Yevon) were more a force of nature than anything else. They weren't out to destroy the world or anything.

No, Yevon had originally designed Sin to be a super-weapon, not unlike Vegnagun. Unfortunately, he miscalculated and ended up losing his humanity and all control over his actions. And destroying the world would've ed up his true goal, to preserve Zanarkand's memory forever through DZ.

What little damage they did to human society was just an unfortunate consequence of their existence.

Not true. Sin's destruction was entirely intended, just not 1000 years' worth of destruction.