I love the Fray's "How to Save A Life". It was an instant hit with me and I keep playing it on bad days and it helps keep me going. Chasing Cars took me awhile to like, but I eventually did.
Easy choice for me.. "Chasing Cars". That's one song I can never get sick of, and "How To Save A Life" is so overplayed I wouldn't care if I never heard it again. :P