As much as I love Dark Side, I have always loved Wish You Were Here just a little more. Shine On You Crazy Diamond is just so exquisitely composed, and the way it wraps around makes it perfect for looping the entire album and just listening to it over and over again.
On a related note: I highly recommend the Classic Album episode covering Dark Side of the Moon. I've seen many other Classic Album episodes (Sabbath, Maiden, Rush, Deep Purple, etc.) and none of them are as fascinating, from a musician's standpoint, as the one for Dark Side.