"Comfortably Numb" is definitely up there for me, but my absolute favourite at the moment is "Us and Them". Just beautiful. Other close contenders would be "Still Haven't Found What I'm Looking For" by U2, "Mr. Tambourine Man" by Bob Dylan, "Nights in White Satin" by the Moody Blues and "Aqualung" by Jethro Tull (Thick as a Brick is awesome, too ;)).