It's so hard to let someone go when you truly love them.

Her and Cinderella their dreams after all these years come true

All these moments are so nice

This truly marks the change in the Beast. He loves Belle enough to sacrifice himself breaking the spell. I honestly think if he HADN'T let Belle go, she never could've loved him back and the spell wouldn't be broken anyway.

Letting her marry who she wants and Letting her leave to go help her Father as well.
But fairies have saved Aurora's life!
Well, yeah, maybe it's not the SWEETEST... !?
Then definitely that the Beast has done.

and Beast's.

Seriously? The beast? Saying "i'm not going to keep you here against your will anymore" is the sweetest thing done for any of the princesses?!?!

Right. No. I'll have to disagree, that's just decent behaviour, really just above the bar for not being a crappy person.

Triton turned his youngest daughter into a human so she could be with man she loved even although that meant she wouldn't be part of the merworld anymore.

It was great that the Beast has done. Here's the script:
BELLE: Papa. Oh, no. He's sick, he may be dying. And he's all alone.
(BEAST turns, then looks at the rose, deep in thought.)
BEAST: Then...then you must go to him.
BELLE: What did you say?
BEAST: I release you. You are no longer my prisoner.
BELLE: (In amazement) You mean...I'm free?
BEAST: Yes.
BELLE: Oh, thank you.
(To MAGIC MIRROR) Hold on, Papa. I'm on my way.
(BELLE turns to leave, then turns back and pushes the MAGIC MIRROR back to BEAST.)
BEAST: Take it with you, so you'll always have a way to look back, and remember me.
BELLE: Thank you for understanding how much he needs me.
(BELLE turns to leave and BEAST looks down in depression. She touches her hand to his cheek and rushes out. We see BELLE's skirt fly past COGSWORTH, who has entered the room.)
COGSWORTH: Well, your highness. I must say everything is going just peachy. I knew you had it in you.
BEAST: (Very sad) I let her go.
COGSWORTH: Ha ha ha, yes. Splend--
(COGSWORTH stops in the middle of his sentence.)
You what? How could you do that?
BEAST: I had to.
COGSWORTH: (Still amazed) Yes, but why?
BEAST: Because, I love her.

^OF COURSE it's one of sweetest things here!

They're all so sweet. I would say this one because I am so dang selfish. I'll give the Beast credit for this one.

I agree with SG1-090. The Beast one isnt sweet, its just common decensy. You know? What ANY normal human being would do.

also human, ball, and "be willing to marry someone else so she can have her dream", as Naveen did so Tiana could get her restaurant.

The Beast's is sweet....imagine you having to let go of your true love never thinking you will see them again and losing all hope of ever becoming a human again and staying in the beast of a hideous creature locked in a dark castle where no one ver shows up just to make HER happy so she can help he father <-----IF that is not sweet i dont know what is because that is one of the hardest and most selfless things to do especially when you are an ill tempered beast who was just beginning to hope.

well said, TotallyMe105!

Yeah, I'm really selfish. That would be a very hard thing for me to do.

I think Beast's ISNT sweet. He was keeping Belle prisoner. He was SUPPOSED to let her go, morally.

beast was thoughtful to let belle go to her father.
my second pick was ariel's dad letting her become human.

Triton knows he'll never see Ariel again, but makes her a human anyway because he knows that is what she wants. It was a completely selfless act.

I cried, to me it was by far the sweetest. I don't care what he did before then, he changed. <3