Ulfric is High King by right. Now before I continue, let me just say - I am not alleging myself with either particular side of the civil war. I'm just stating facts. Facts that you can't ignore regardless of your personal opinion of Ulfric or the Stormcloaks.

So let's begin; In the ancient times, Nordic custom was that one man can challenge another to an honorable duel. If both men agreed, they can meet up and fight. The first to give up or die was declared the loser, and the winner was given whatever they were dueling over.

"But Ulfric walked in and killed him"
This is completely false. In game dialogue does prove that Ulfric challenged Torygg to an honorable duel, which Torygg accepted.

"Torygg had to accept, he didn't have a choice"
Yes he did. As any man has the right to challenge someone to an honorable due, any man also had the right to refuse a challenge,

"Torygg would of looked like a coward if he rejected"
Doesn't matter, he still accepted. He just put his pride over his life.

"Using the thu'um was cheating"
This is bullshit. At the time these customs took effect, using the thu'um was not only allowed, it was praised. Even a basic shout like 'Fus Ro Dah', takes years of meditation and dedication to be able to perform. Using the thu'um was just using what you know, and it served as a testament to your dedication and strength.

"Torygg didn't know Ulfric had the thu'um"
Complete fabrication. Not only is it common knowledge that Unfric studied with the greybeards. Ulfric is a well known war hero. And it's well known he used the thu'um in the war. If Torygg didn't know, that's his fault.

"The Empire does not recognize this law"
There are no records of the Empire changing this law. I could understand why they wouldn't give Ulfric the throne, but charging him with murder while they watched Ulfric duel and kill Torygg is just ridiculous.

"This custom affects the smallest things, and no the throne"
False. Jorunn the Skald-Kind challenged his brother to the throne, and got it when he won the duel.

Now:

"The rebellion is stupid, because it divided the Empire, and Skyrim will fall to the Thalmor. With the Empire in control, they have a chance to resume war with the Thalmor. Not to mention Ulfric is a racist"
I'm getting tired of healing this. But especially the "Ulfric's a racist" thing.

First of all, just to get it out of the way, I'm pretty sure true Nords would rather go down fighting the Thalmor than living under their rule.
Second, "they have a chance to resume the war." The war shouldn't have stopped in the first place. The Empire was winning, and they just surrendered.

*Joanna's battered legions. In the end, the main Aldmeri army in Cyrodiil was completely destroyed, The Emperor's decision to withdraw from the Imperial City 4e 174 was bloodily vindicated.*
So even if the Stormcloaks were to lose, it's more than the Empire would do.

"But Tullius said they are going to fight the Dominion?"
Well, Tullius doesn't run the entire Empire, does he?

Now onto the racism. Ulfric and the Stormcloaks are not racist. That's a very big accusation. If you think Ulfric and the Stormcloaks are racist, you have not been paying attention. You need to open up your mind on this a bit. There are only two real racists in Windhelm, Rolff and Angrenor, who were first seen bullying a Dunmer woman when you first enter the city. Other than them, what makes Stormcloaks any more racist than anyone else? And seeing people immediately changing factions just because of these two fuckers makes me cringe. So I really need to get this off my mind.

First of all, if Ulfric truly was a racist, then why would he let Altmer and Imperials live nice lives within his city and lands? If he really was racist, you would think he would be treating the Altmer (Race of which the Aldmeri Dominion is centered around and consists mainly of) and the Imperials (Race of which the Imperial Legion is centered around and consists mainly of) worse than the Dunmer and Argonians. But there are Altmer and Imperials living wonderful lives within his city (For example: Niranye and Viola). This is because, unlike the Dunmer in the Grey Quarter, they actually contribute (I'll touch on that as this goes on).

Speaking of the Dunmer, many Imperials also like to use them being in the Grey Quarter to further their argument. But have you not noticed Belyn Hlaalu? He's a Dunmer who lives a nice life on a nice farm on Ulfric's land. If he wanted to, Ulfric could take that farm from Belyn with a snap of his fingers. Belyn contributes to the Stormcloak cause in Windhelm. This is proved when he says "The only way to win the Nord's respect is through hard work." Indicating that he is working for the Nords who occupy Windhelm, aka the Stormcloaks. He also says "Too many Dunmer complain about the way we're treated. What good does complaining do?" I agree. I don't see those Dunmer in the Grey Quarter doing anything to help out, if they did, they would be living a life similar to that of Mr. Hlaalu. He helps out the Stormcloaks, so he gets to live a nice life. Have you also considered that Ulfric is a bit busy with a war and then preparing for a bigger war to spend a large amount of money on renovating a quarter of a city filled with people who don't even contribute?

Now moving onto the Argonians.

You also see people condemn Ulfric for keeping the Argonians out of Windhelm. But last time I checked, Brunwulf Free-Winter (Whom becomes Jarl should the Legion win) also keeps them out and his reasoning is that "they must remain outside for their own safety". Have you not considered that maybe that's why Ulfric keeps them out? It appears not. Nothing changes for the Argonians when the Imperials take over. Does that make the Imperials racist as well?

Ulfric respects anyone who supports the Stormcloak cause, regardless of race. Also, let's not forget the fact that Nords are a naturally proud race, but that doesn't make them racist.

And finally, to the people who read the Thalmor dossier on Ulfric Stormcloak and immediately assume he's a spy.
So many people read that and don't understand what it is saying. They call him an asset because his actions are inadvertently helping the Thalmor. They captured him and tortured him for info during the last war. Then they let him escape so that he could stir up trouble with his unrest over the Talos ban. Does no one read the "uncooperative" part? It means he's not trying to help them. His actions just happen to benefit the Thalmor since a civil war gives them time to gain strength and take over at a later date. But if the Stormcloaks win it will still be bad for the Thalmor. The dossier says as much.
People who say Ulfric is a spy really don't understand the context of that report.

I hope this helped to enlighten whoever read it at least a little.