i can't believe Mulan is winning....?

^

Because she admitted she did it in part for herself, while the other two had nothing at all to gain from it.

if they meant the LEAST hard sacrifice, I'd go with Belle's though.

^ Exactly that. Belle basically signed her life away and Anna is waaaaay to selfless for her own good.

Because it wasn't just for her father, it was for herself as well.

Well I don't think she knew when exactly she was going freeze completely, maybe she could have saved Elsa and Kristoff would have reached her still in time.
About Mulan, it's not like she still would have wanted to go a war even if she said it was to prove herself she could do something right. And what she exactly tried to do right? Go to fight in a war and maybe die in place of her father so he could live. Like in a way she tried to have honorable death, it's not any dream she tried to complete,it was pretty desperate last thing she could have done.

This the hardest questions EVER

Mulan and Anna could have died! I don't really see a problem with Belle.

She also could have gotten her family punished if found out.

Tough choice really, but I think Anna is a little less than Mulan and Belle.

Basically, her. Anna DIES, thanks to protecting Elsa. We see Anna's last puff of air leave her mouth.

Mulan sacrifices for her father, but doesn't know if she'll ever come home alive to her family, since it is war. Mulan risks her life to rescue Shang when the avalanche threatens to suffocate him. Mulan also mounts a rescue team to get the emperor away from Shan-Yu.

Belle switches places with her dad one time, then doesn't do anything sacrificial after that. The answer is Belle.