When I first saw this thread I thought it meant singers *behaving* badly and I was going vote for Joe Cocker and Jim Morrison. Now Val Kilmer was pretty good in movie of Jim Morrison behaving badly but I don't recall if he did any singing. Anyway, actors singing badly is a bit off track and maybe that's a whole other thread.

Although, Alice, I thought the Monkees were pretty good singers for actors.

Duckboots