Howard Morris (The actor that played Ernest T Bass) would have made a good Joker.
Morris had the laugh, but not nearly intimidating enough. The Riddler, maybe.
You missed Tom Bosley, Steve Buscemi, and Normal Fell.
By the way, am I the only one who misread the thread title as “John Denver as the joker”?
He already played a supervillain (Cockknocker) in Jay and Silent Bob Strikes Back.