It isn't about who can kick who's ass. It is about who is the best developed or possibly evilest villain that there is. I don't think that Godzilla is capable of knowing what is considered right or wrong by humans, and thus to me, Godzilla isn't that great of a villain... Just a big fat treacherous dinosaur on steroids.
I would have to disagree with Vader. While he is a remarkable character, I wouldn't classify him as the best villain because of the conversion he makes near the end. For the first two movies (Episodes 4 and 5) he is an incredible villain, but not so much in the third.
I liked Agent Smith in the first movie specifically,beacuse he was the total opposite counterpart of the savior of the world, Neo. Smith was just so... evil! Hugo Weaving was brilliant as Smith in the first movie.