i think All Along the Watch Tower by Jimi Hendrix was probably one of the best covers I can think of, especially because it was greatly improved from the original. Kudos to Dylan, though.

Anyhow, I've never heard any Pink Floyd covers, aside from Wish You Were Here by John Reznik and Fred Durst...and I hated that. I've heard about Another Brick Part Two floating around somewhere featuring Tom Morello, but never heard it.

the I Am Sam soundtrack has some good Beatles covers, as well.