Definitely Stefan. Damon is a selfish person that kills people whenever things don't go his way. Yes, Damon does love her, but he shows it in a very different way. He's alot more rough with Elena than Stefan is. Although throughout Season 3, he helped her save Stefan.
Stefan on the other hand was always kind to her since the beginning. I mean, yes in Season 3, he did go off the deep end, but that was only because Klaus turned his feelings off.