Oh, so many stinkers, so little time (and thank goodness *I* don't qualify as a "respectable artist!"). Let's see:
Judy Collins' version of the Stones' "Salt of the Earth," topped only by
Joan Baez' version of the same song (Baez, whom I normally love, is a possible candidate for the Worst Cover Versions Hall of Fame, especially for her stilted readings of rock songs and politically correct butchery of lyrics).
Special Blackmail Material Edition: the live duet of Bruce Springsteen and Van Morrison drunkenly blundering their way through "Brown-Eyed Girl." I often listen to it to remind myself that even the greats can have a reaaaaallly bad night.
Judy Collins again, for "Both Sides Now;" although it's not entirely her vocals' fault--it's that dripping-with-treacle string-and-keyboard intro and arrangement.
The Four Seasons' (aka "The Wonder Who") 1965 cover of "Don't Think Twice, It's All Right" (though an argument can be made that it's so deliberately bad as to be hilariously campy).
"Endless Wire," by Gordon Lightfoot. Sings the entire song, especially the bridge, through his nose to the extent that the lyrics are utterly unintelligible.
"Touch Me, Babe" by the Doors. Inane lyrics, stupidly poppy horn section. Only good part was the "stronger than dirt" at the end.
"Coconut," by Harry Nilsson......no, wait a minute, that's so stupid that it's a guilty pleasure. I think it must have inspired "The Song That Doesn't End."
"Who Will Save Your Soul?" by Jewel. Okay, so she may not be "respectable," but those idiotic vocal mannerisms make me cringe. Come to think of it, just about ANYTHING by Jewel.....
And speaking of annoying vocal mannerisms, I'm gonna get flamed for this, but anything Ani DiFranco released after 2000.......
And the Grand Prize goes to (drum roll....................................................);
Chuck Berry, for "My Ding-a-Ling!"