Imperials.
Ordinarily, I'd side with the Stormcloaks, but there is one key consideration here, and that is the fact that the Stormcloaks are being backed by the Thalmor.
'What,' you ask, because I'm assuming you don't know this because why would you even consider the Stormcloaks otherwise, 'why would the Thalmor back the Stormcloaks? The Thalmor hate Talos, and the Stormcloaks ride his dick like it's going out of style!'
Great question!
Read
this (it's like 2 paragraphs, come on).
Well, shit. The Stormcloaks are useful idiots to the Thalmor because they weaken Imperial authority, and by asking for and receiving
official permission to worship Talos, the Imperials have to stop closing one eye everywhere else and properly enforcing the terms of the White-Gold Concordat in those places, which means less Talos worship overall.
But why do the Thalmor even give a shit about Talos in the first place?
To very briefly summarize, those fuckers want the end of not just the world, but the universe, because then they can become immortal, because they think Lorkhan stole their immortality, and-you know what, fuck it, just trust me on this one. If you want an explanation instead of the vaguely stated 'trust me' that generally indicates 'I am not trustworthy', I'll attempt to prove trustworthiness by my sources, and say that
here is probably the most succinct summary you'll get, otherwise start
here for an interpretation, or
here for some stuff on Vehk, who is key to the concept of CHIM, or
maybe here if you want to delve into it at the source, because frankly this shit hurt my brain the first time round and I don't know if I ever recovered.
Oh, also they're racist against humans, but, I mean, okay they have a decent reason to be, man and mer have some pretty serious issues to work out. People broke time for a while trying to remove the elfiness from a god, there's been lots of genocide both ways, it's like two dudes who hate each other locked in a room, where every day one gets a gun and as much ammo as he wants, and then the next day the other one gets the gun and ammo instead of the first guy, and also they both keep coming back to life I guess, wow this was a shitty metaphor, but forever. It's a shitty metaphor, forever.
Metaphorever.
That's how much CHIM broke my brain. I saved you that pain, people.
Tl;dr Stormcloaks are stooges of universal nihilists, murder Ulfric and support your local Imperial Legion today in stopping criminal scum
right there