Not so sure about the pieces you cite (though I happen to like them) as "classics" - but his "Jet Plane" certainly qualifies as influential, providing PP&M's first big hit. Personally, I didn't get fed up with Denver until he came down with the terminal country-boy cutesies. "Poems, Prayer, and Promises," "Aerie" and "Farewell Andromeda" have some pretty good stuff. Darcy Farrow, Casey's Last Ride, some Prine (Spanish Pipe Dream and Paradise in there somewhere). He got wrapped up in an image and lost some of what infused his early music.