My taste runs toward the esoteric, so there are lots of very popular bands I don't like in general, but Free Bird (or Sweet Home Alabama) would be strong contenders for bad-but-extremely popular songs. I'm going to pick a solo work by Stephen Stills. I'm not fond of him but he's been a part of other groups who made some great music and wrote some decent songs with them (e.g. "Rock and Roll Woman"). "Love the One You're With" has enough mediocre goodness that it's not an abhorrent turkey, but it's smug and catchy in a bad way, and nobody but me seems to mind it.