The Liverpudlian psychic Derek Acorah.
Well apart from the Dragonborn and his dragon allies (Ulfric and Tullius can kiss my butt!)it could be:-
THE NORDS,and the citizens of Skyrim but there are better ways of making it so than civil war. The Empire ruled by consensus not by conquest. The deal with the Altmer has stuck in the throat of many people within and without Tamriel. The milk drinking emperor has in effect undermined and dissolved the Empire by his actions. All satellite states such as Elswhere and Valenwood (already purged), Hammerfell, Skyrim, High Rock, etc must look to their own future and decide whether they want to replace the Imperial controls with Altmeri ones. Hammerfell has said no and Skyrim must do the same. Without that stance any chance of a level of autonomy and self governance, protection of traditions and religious beliefs etc is lost. The Imperials showed respect for an honoured Nord traditions and beliefs. The Altmer have no intention of doing such a thing. The choice is simple a declaration of independence or total submission as a second class citizen or slave.
OH bother! I've taken this as a serious question
I suppose it belongs to both the Nords and Imperials. Since the Nords were never fully conquered(There was one major battle, the Nords lost that and then joined "voluntarily''), the Imperials can't fully claim it by right of conquest, the Nords do, but it's still part of the Empire, so it also belongs to the Imperials. Unless the Stormcloaks win in lore (wich I doubt, the only way the Empire would let go is if Skyrim would remain a vassal).
The Falmer lost Skyrim long ago, they don't own it anymore, but still have a good claim to take it back rightfully.
I think the Eye of Magnus, what ever it was, was the last straw for the Snow Elves. Before that, the Atmorans were making babies like rabbits, and the Elves just couldn't keep up. I think with the discovery of the Eye, it was just to much of an advantage and they made a preemptive strike at Saarthal.
If they hadn't done that, the Snow elves might still be around. Oh the decisions me make.
Once the Falmer laid down the ground rules for war, the Atmorans played by them. They beat them at their own game, and the name of the game is genocide.
I don't fault the Nords at all. When it becomes driving out a people, you can't leave children. It's awful, but that is the best way. Kill children, and you win battles you'd have to fight in the future.
Best claim is Falmer.
But, Skyrim belongs to the Empire right now. Not to the "Skyrim belongs to the Nords!" barbarians.
EDIT: Ah, and I forgot to mention... I don't think Skyrim "belongs" to someone really. It's only on the man/mer/other races's perception that this concept of owning exists.