There are so many to choose from. My favorite songs when I am feeling down are:
1. Shelter from the Storm by Bob Dylan - I actually wrote a hub about this one.
2. Can't Get It Right Today by Joe Purdy
3. Hallelujah by Jeff Buckley.
4. The Scientist by Cold Play
5. Bridge over troubled water by Smin & Garfunkel
I could probably go on and on but I think those are the best of them.