Well BGD, you're definitely right about those four ... wow! I'm not really into blues, so wasn't aware of those (thanks for the effort!).
The covers I mainly had in mind are more from the classic rock / prog rock scene, and most of the ones I can recall are awful. I've heard a few versions of some Jethro Tull tunes, and they don't work at all. And I had just heard the Bare Naked Ladies version of Bruce Cockburn's "Lovers in a Dangerous Time", and that's what prompted my question ... it's terrible. BNL are a band I usually like a lot, and most of their original stuff is great.
And EasyBeat, you're right about Cocker ("I Get by With a Little Help From my Friends").
I'm glad I asked the question, and would love to hear some more examples.