The Stormcloaks. Sure, Imperial dogs make claims about "We have to stand together" and "Nationalism" and what have you, but here are the big points to me:

1. Worship of Talos. If your 'nation' has signed away your right to worship, they have nothing worth saving and don't deserve your fealty.

2. Punishment of the accepted Nord forms of succession. Using your increased skill to your advantage isn't 'cheating' just because the guy the other guys likes lost.

3. The Empire is LITERALLY going to kill you at the beginning of the game. As in, you are in line for the chop. And you are supposed to just...handwave that away? I mean, sure, you begin Oblivion and Morrowind in jail- but there is a BIG difference there.

So, in conclusion, Skyrim is for the Nords, Ulrich did nothing wrong, and the Stormcloaks are 100% the right choice.