Anyone ever heard Oh Yeah! by Charles Mingus? It's got him and Roland Kirk (as he was then) on. It doesn't seem to be as well-known as say The Clown and Pithecanthropus Erectus, Mingus Ah Um, Black Saint etc...; I only bought it today, but it's amazing, and is fast one of favourite 3 jazz records of all time. Mental, but in a good way.
Another example, this Donna Summer track is the best thing she ever did?: http://www.youtube.com/watch?v=Kzbd4ILqwkw
Now your turn (hopefully), what forgotten classics exist by people most people have heard of?/or haven't, I don't really mind.