Well, White Man can't come into the equation as it was never on an album, but I agree that it is hard to argue against it being their best track.
Lewes Punk mentioned Lost in the Supermarket - to me this is one of their greatest songs. Revolution Rock, Clampdown, Rudie Can't Fail, Train in...
Disagree completely.
London Calling is a sublime album fro start to finish, and remains one of my favourite albums of all time.
I saw The Clash many times live, and love most of their output. But London Calling was their masterpiece.