and Stefan. Elena can't even chose between brothers for her OWN good. And Damon... not so much.
:/

For me the winner is Elena. She was ready to die to kill Klaus, she "died" saving Matt, more than once she made risky deals with Originals & put herself in danger becasue she believed that was better for her friends. She's definately incredible selfless & ready to sacrifice everything for the ones she loves.
Then DAmon. Damon. [copying my comment from TVD spot]Until season 3 he didn't really fight for Elena. He stepped back, becasue he felt Stefan & Elena deserver each other, wherease there's no place for him by Elena's side. He sacrificed his happiness becasue he thought he's not good enough to make Elena happy. That's one thing.
Other one is the fact that he's always doing what has to be done, even if that meant that everybody will hate him after that: he faked Bonnie's death, he turned Abby into a vampire, he tricked Rebekah to learn about the Original Tree, he compelled Jeremy to leave the town - all he gets is everyone's anger.
Stefan... Ok, Stefan gave himself to Klaus to save Damon, that's huge & I really appreciate that. But while Stefan did one huge thing. Damon is giving up his happiness time after time to do the right thing.
I wouldn't reallt say Bonnie. She lost the most, that's true. But all of this was just taken away from her, she didn't really "sacrifice" it, she wasn't ready to gaive it all away - her greandma, Luka, Jeremy, Abby. Poor Bonnie lost it all becasue of all supernatural staff going on in Mystic Falls. But it's true she was even willing to die for Elena, to get rid of Klaus.
They are all selfless & ready to sarfice themselves, but my personal list is Elena > Damon > Stefan > Bonnie.

She is the most self-sacrifacing, in a bad way, she's more like suicidal. She always makes deals that might get her killed, without a second thought.

And Stefan!

And Damon and Stefan a little bit. But Bonnie? Just no.

By far
Then Damon

In S2 I loved her for it. Last season, I wanted to slap her.

And Elena