I can hardly believe in 20 pages of posts, this one hasn't been mentioned yet:
As Carlisle is treating Bella following the attack by James..
Edward's face was drawn. I watched his eyes as the doubt was suddenly replaced with a blazing determination. His jaw tightened. I felt his cool, strong fingers on my burning hand, locking it in place. Then his head bent over it, and his cold lips pressed against my skin.
"Stay, Edward, stay with me . . ."
"I will." His voice was strained, but somehow triumphant.
I sighed contentedly. The fire was gone, the other pains dulled by a sleepiness seeping through my body.
"Is it all out?" Carlisle asked from somewhere far away.
"Her blood tastes clean," Edward said quietly.
"It was impossible . . . to stop," he whispered. "Impossible. But I did." He looked up finally, with half a smile. "I must love you."
Twilight, pages 456 and 460
True, deep, crazy love. Making the impossible, possible. I think this was my favorite moment in the entire series (but I have a feeling Midnight Sun might change that..)
As a side note, I couldn't fully enjoy the movie version because I think the screen version totally ruined this moment. Edward accomplished this amazing feat all on his own, fueled by his incredible love for Bella -- no help from Carlisle necessary.