That's a tough question but since Xemnas was able to make two of himself in the last battle on kH2, Vader might be in trouble. It also depends on whether or not Vader can use his ships and storm troopers and whether or not Xemnas has himself backed by the Organization. I think Xemnas would win.
I would think it would be one on one... otherwise, Xemnas has the Org, that Dragon thing, and an endless supply of Nobodies. Not to mention two of himself. lmao.
Psh. Think about that. Vader is just good with swordsmanship... and he can move/shock/choke things with the dark force. But come on. Xemnas is a million times faster, and can do the same things... but far greater. Vader has one light saber. If that's gone, he's done... Xemnas can use two at a time and summon them as arrows and everything. Then there's cloning and his god-speed...
Gotta love Darth...but still Xemnas would win hands down....like liso said, Xemnas has the Org and two lightsabers. Darth has only one and is like 50 years old. (No offence to George Lucas or Star Wars fans in any way...thank you)
Xemnas all the way.
Darth would win i mean come on dont think age if you take then both at their prime then Darth would win. I mean his force powers are greater than almost anything else. Plus if the go hand to hand or should i say lightsaber to lightsaber darth vader would win cause he is far more skilled. He fricken fought some of the toughest jedi's in the universe and won. Plus if you do think army wise Xemnes has heartless but i beat he cant bring in enough heartless at once without wearing himself out to much to defeat a army of Storm Troopers and the loads of other vechicles at his display