The Dominion, easily.
The Nords are weakened by the Civil War. Even if they united for a full-scale war against the Thalmor, they would have thin numbers and low supplies.
The Thalmor are highly skilled in magic, unlike the Nords. This means they can use magical protection and offense as well as physical. It's likely that the Nords would also have magical aid, but it would be much less potent due to a lack of mages.
The Aldmeri Dominion also includes the Bosmer and Khajiit. Meaning they could potentially have an army three times larger, possibly more with the Nords lack of numbers.
The Altmer have much longer lives than the Nords - or any other race for that matter. That gives their soldiers and mages many more years to master their skills in battle. Physical strength doesn't always make the winner in a battle, but instead training, reflexes and intelligence. You could give a sword to a 1970s Arnold Schwarzenegger and he would be intimidating, but without training he would be useless in a battle.
The only way I think the Nords have a good chance would possible be in a naval battle, but even then it's difficult to say. The Nords are very strong at sea, but so are the Altmer.