ANYBODY singing "Proud Mary". Mostly a matter of personal taste, but I HATE that song.
My acoustic music club insists on playing "Men of the West" ("Rosin the Bow") slowly and sadly, and it just makes me want to scream.
The whole of the Willie Nelson album that included "Sentimental Journey" (I can't remember the name of the album). I love Willie but I got the CD for my birthday, listened to about 15 seconds of each song, and sent it off to Goodwill.
The best version I ever heard of "Achy Breaky Heart" was a zydeco adaptation. It was in Cajun French, which I can't understand . . .
Steve Earle has written a considerable number of good songs, too. I would say at least as many (percentage-wise) good songs as his idol, Townes Van Zandt, who was often good but also suffered from questionable taste ("German Mustard", "Silver Ships of Andilar", lots of stuff about cheap women).
Hank Williams, Sr (another artist I otherwise adore): "My Bucket's Got a Hole in it", "Setting the Woods on Fire".
OK, it's not folk music, but I'm going to duck and run after I write the following: I saw Charlotte Church, the teenage Welsh soprano, on TV a while back and she did a version of "Summertime" that made me change the channel. I don't mind the song and I certainly can't criticize her voice, but the combination of pseudo-blues and opera just didn't work.
*whew* glad to get all that off my chest