To be honest I can't decide on just one. I love them all to put only one as 'the best.' In each song, she portrays it fantastic and fits in the style of the music. In love story, she's a beautiful Juliet needing her Romeo. In you belong with me, she plays a very believable unpopular girl. Each song she has done, she fits her personality in it somehow and she sings with joy, love and passion for each song.
AAH! Love Story is so overrated! It's so annoying how people say that it's Taylor's best song! I mean yeah, it's amazing, but there are lots better!
And I think "Ours" it's really cool... too bad that it's not really popular :(