Jimi Hendrix was also a 'one hit wonder', given that his only hit was with his cover of Bob Dylan's "All Along the Watchtower", but, like Denver, his influence on music (and indeed, his talent), went far beyond any pop-chart success.
And reading that makes me wish I'd bough the John Denver vinyl records I saw at the thrift store, so I could send them to ya, Kiro. Maybe if they're still there next time I go, I will.