What are the most covered songs in the past fifty-ish years?

I'm not really interested in clical music here, as I'm sure old stuff like Canon in D has been remade thousands and thousands of times. I'm looking for bonafide recorded covers with widespread release, such as "All Along the Watchtower" (Dylan, Hendrix, U2, DMB, and more) or something. I just searched iTunes for Message in a Bottle and was amazed, so I thought I'd ask - thanks!