Spoiler ulfric is a thalmor sleeper, everything he does indirectly benefits the thalmors ultimate goal of keeping the empire weak and vulnerable, the thalmor even secretly cede carefully limited funds to the stormcloak rebellion, ulfrics markarth rebellion was based off false information spoonfed to ulfric during interregation unbeknowest to ulfric that the interrogators were fully aware of his listening in and what he did in markarth is exactly what the thalmor wanted him to do
I've seen no evidence to support that claim. What you have is a dossier of the Thalmor which claim that he was an 'uncooperative asset', which is to say that they thought they could profit by manipulating him into doing things which might further their aims. They captured him, then let him go betting that anything he would do would play into their hands.
That doesn't make him a Thalmor agent, that just makes him bad news for the Empire. Well, guess what: When the Empire agreed to outlaw the worship of Talos, they signed up for a lot of bad news.
I support the Imperials already, because they have the Empire bound together at the moment, and keep it strong. But I also have supported Ulfric, seeing how the Imperials have not exactly been good people to the Nords of Skyrim. All in all, both sides are gray and both sides have the same goal; survive.
Ulfric's goal isn't survival, it's to live free or die trying.
You want to take the measure of each side? I give you Benjamin Franklin:
"
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety."That tells you everything you need to know about Ulfric and the Empire.