It seriously changes everyday, with each mood I go through, but an all time favorite would be Don't Panic. Helped me through a lot, and it has so many memories attatched to it.
Another is Rush of Blood to the Head, since it was the first song of their's I ever really listened to and loved.
U2!!!, The Beatles (solo or together), Led Zeppelin, the Rolling Stones, the Doors, the Arcade Fire, Pink Floyd, the Who, Bob Dylan, Hendrix, Creedence Clearwater Revival... I could go on, but that'd take all day.