Artists Seemingly Incapable of Laying a Turd

Iris Dement, Buddy Miller, Emmylou Harris, Rodney Crowell, Patty Loveless, Jim Lauderdale, Rhonda Vincent, Ricky Skaggs, Bonnie Raitt, Ry Cooder, Richard Thompson, Loudon Wainwright III, Leonard Cohen, Hank Williams, J.S. Bach. John Hiatt, almost ;-).

Any Quartet with a lead singer named Jim Morrison.

I can't think of an artist I love that hasn't laid a couple of eggs...but I'd say Vladimir Horowitz has come closest to the ideal.

Leonard Bernstein tried to be in jazz and did not succeed. Everything else pretty much is extremely great and inspiring including lectures.
I would assume that in his opinion Boston Pops is a turd. 

Bruce Cockburn