Favorite album? Hard to pick just one. If pressed, I'd cite 1972's career-killer Son of Schmilsson, and I also have a great deal of fondness for 1974's Pussy Cats with John Lennon.
Underrated? I'd say 1976's Sandman, but others may disagree.
At any rate, he left us far too early (he was only three years older than I am now when he passed in 1994!), and I wish the people in charge would get the documentary, as well as the album he was working on when he died, out in my lifetime.