Which entity of the two, a Stormcloak Skyrim or a United Empire , would have the best chance of militarily defeating the Thalmor lead Aldmeri Dominion?

Actually I'd prefer the third way - an alliance of independent nation states. Sadly this isn't an option in the game, and in practice it would probably be an uneasy alliance at best, given the nature of Skyrim's bid for independence, but I still think it would stand the best chance while not being, you know, an empire, I'm not fond of those. (Being British and knowing how awful ours was, even when it supposedly meant well.)

An-Xileel. Argonians are currently at the height of their power, and the hist can practially predict any event, and we already know that argonians are impossible to defeat or conquer on their home turf ( only coastal towns near border but the inner swamps are way too dense for anyone but argonians ) This gives them a stable base of operation to launch endless swarm attacks. Basically they are the closest thing to Tyranids from warhammer 40k, in a way that they can be connected via hive mind and genetically altered where you'll have 7-8 foot argonians and over 10 foot tall nagas and so on.

The only issue I see is maybe the hive mind link weakening due to being far from Black Marsh, but im sure the hist would be able to compensate somehow for this. Given how easily they smashed the daedra, I see them as only viable option as of now to defeat the Thalmor.

A single Legion and one competent commander all but decapitated the Stormcloak movement until a dragon showed up, butchered the Legion veterans who were fighting it (and guiding the civilians to safety) and allowed Ulfric to escape.

Basically, an exhausted Skyrim under the command of a petty warlord with ego issues and deeply buried PTSD is buggered pretty hard. A single legion that was forced to recruit from the locals instead of bringing in their trained, experienced forces was stalemating Ulfric.

Nords in skyrim cant even unify on this civil war issue, plus the Empire has more resources

I'd find an army to defeat these "Stormcloack-vs-Imperial" threads and make them stop, once and for all.

Oh I would hardly call it impossible.And endless swarm attacks?I think Argonians need to breed and it takes time.They aren't a giant hive organsim that can recycle every dead into a new soldier quickly.And apart from that part in the books there is no source that they can alter them that way and that was also not a normal Hist.

As others have said, a united Empire has a far better chance of defeating the Dominion than if Skyrim broke away. Ulfric also doesn't stand any chance of taking on the Dominion alone, which he will have to at some point.

Psijjics. The Thalmor will be taken down by Altmer who do it better than they do. They now have control of the Eye of Magnus, and who knows what else that they've collected over the centuries. They've been studying magic for longer than anyone else, and that's the real source of power on Nirn.

And the Psijjics aren't a threat to anyone else, because they prefer to stay private, and study. They'll only intervene when the Thalmor directly threaten them, but of course they will at some point, because that's their nature.

The Stormcloaks wouldn't stand a chance against the Dominion. They'd be too busy in civil war #436 where half of Skyrim is fighting the other half of Skyrim because the king of Skyrim was killed in a legal duel (the 9th king to die in a legal duel in just 2 weeks).

Even if Ulfric and the Stormcloaks were leading the fight against the Dominion (which they wouldn't be because the next war will come many years from now, Ulfric will already be long dead from old age) they wouldn't be able to defeat the Dominion. The Stormcloaks have no talent for the bigger picture. They're too focused on tradition and petty tit for tat squabbles.

As put, the answers has to be the Empire even if it's based on sheer numbers alone. But the Stormcloaks don't want to destroy the empire and fight the Elves on their own anyways. They are happy for the empire to exist and rule, just not in skyrim. If Ulfric and Tullius had any sense they'd team up and fight alongside Titus Mead's Cyrodiilic army to take down the dominion once and for all....... Then restart the civil war squabble. But their strategies are all ego driven. Though I get the impression that Tullius would just as soon retire by the sea!

If Wuunferth the Unliving would get busy and whip up a bunch of Resist Magic amulets and rings for the Stormcloaks they might have a chance. From what I've seen in game when 3 Stormcloaks go up against 3 Thalmor they get lightning and fireball spammed so fast they don't have a chance to close the distance and kill. You need overwhelming numbers, magic resistance, or friendly dragons to beat the Thalmor.