I'm not one to argue over inane definitions of musical genre, but when I think of 'best classic rock album', there's really only one thing that springs to mind: In the Court of the Crimson King, by King Crimson.
Yeah yeah, it's 'progressive', blah blah whatever. It is a rock album, and it is undeniably a classic. It is also completely awesome, especially for its use of the mellotron.