There is no clear cut good and evil choice, by purposeful design.
If you go with the Imperials, you side with guys who favored appeasement, and who are more than willing to execute people innocent of any offense, just to kill Ulfric and to send a message.
But at the same time, the Stormcloaks aren't fighting for freedom from Imperial tyranny, because no such goddamn thing exists. Skyrim isn't being oppressed under the thumb of the Empire. Fuck, the line of Septims are Nords. Nobody's rolling into town taking their shit and fucking their bitches.
Ulfric is fighting a war based entirely upon personal ambition. He has no reason to kill Elisef's hubby other than the fact the dude is in his way as high king of Skyrim.
To make matters worse, the Stormcloaks and Ulfric are racists. They hate the other races and either want them removed from Skyrim, or want them dead. And the Nord claim to Skyrim is somewhat dubious in the first place. Just look at the Forsworn and King Madanach. They hold a fairly legit claim to the lands of the Western Reach, but the Nords began a violent and bloody campaign to put the Forsworn down. As well as anyone who happened to be standing particularly near.
Ulfric lost me when I overhead him planning to attack Whiterun and burn the place down around the Jarl. I liked the people of Whiterun. It's where Aela hangs out and since she's the only NPC who routinely flashes us with side-boob, that made me suspect of Ulfric. And then, I found out about the racism, and how they made a brutal, ruthless enemy of the Forsworn, it became clear their cause was not just.
The Nords of the Stromcloak rebellion are no more angels than the Imperials.
So, which side is the right side? The side that allows YOU to kill the shit out of everyone who's crossed you. Which is actually NO SIDE, because certain NPCs remain the fuck UNKILLABLE, even after you've completed the Civil Wars quest line.
I would have joined neither side, but a glitch occured that didn't allow me to proceed in the main quest without accepting the Imperial offer to join them. My plan from the beginning of the game was to track down general Tolius and kill him for that shit he pulled in the beginning of the game. You don't put an axe to my neck without me Thu'uming that punk ass. But the glitch removed that choice from me. And I wanted to kill Ulfric for killing Elisef's husband, and for being a generally racist bastich.
But once you choose a side, you can ultimately only kill the leader of the other side. The leader of your own side remains an unkillable NPC. After I'd put the kibosh on Ulfric, I put an arrow into Tolius...and by that time, I'm scoring one-hit kills with my arrows, so that should have been adios muchachos for that nigga. But he can't die. He doesn't even turn hostile.
So it doesn't fuckin' matter which side you choose. Both sides are a box of cocks.
"You must be proud, bold, pleasant, resolute,
And now and then stab, as occasion serves."
Edward II: Act 2 Scene 1, by Christopher Marlowe