Lets examine them all, shall we?

(Haven't beat, so going off hearsay) FF1 - Chaos defeated the original Warriors of light, ruled for 1000 years, and had a religious Cult.

(See FFI) FFII - Emperor ran his empire, conqured a few places, and cheated death only to rule Hell for a brief period.

(See FFII) FFIII - I really have no Idea.

FFIV - Zemus managed to get his seal broken. Really that was it.

FFV - Managed to get his seals destroyed and managed to fuse the worlds together.

FFVI - Kefka took over the empire and ruled over the very world he sent into ruins for one year.

FFVII - Seph killed off Shinra, and obtained the Black Materia.

FFVIII - Ultimecia managed to gain Ellone's power stuff for TC and destroyed Seed in her time period.

FFIX - Kuja sent three countries into War and Chaos to gather the 3 jewels, in addition to getting the fourth from another continent. He obtained Trance, and Overthrew Garland. He also destroyed Terra.

FFX - Came back as a whale every so often and destroyed a few towns.

FF X-2 - Shuuyin went all emo on everyone.

Not sure about XII.

Kuja definately accomplished more, though his reign wasn't nearly as long as Chaos. And Kefka had an entire year ruling over everything directly as a god. Really, as for Achievements it goes to one of those three as far as I can tell.

Ultimecia really didn't accomplish anything accept destroying Seed in her time, and while she gained the power of TC, didn't really accomplish anything with it.

Can't say much about the emperor, though he did seem to do a noteable amount.

Sephy barely accomplished anything noteable. I mean Shinra was replaced within the day. And the meteor plan failed horribly.

Zemus managed to wake himself up. Got a lot of people killed ot do it though, so he gets some props.

Ex-death did quite a bit, though gaining power of the void didn't really help.

And there we go.