Xemnas, mainly because the knight although hard, once his attacks are really like upgraded Org members, they're easy to dodge, once you learn how too, it takes time though and he attacks arn't in order. As for Xemnas in FinalMix Yeah, his attacks are killer, his abilities are killer as well. Though I don't know how it'll turn out if it was KH2 sora in Kingdom Hearts.
Honestly I doubt you can compare who's tougher because of that as well. Due to the fact you can form drive, as well. But I'm sticking with Final Mix Xemnas, cause you could spam heal then, and the battle was even difficult then.