Nirvana - anything. Don't get it, never will get it. I saw that same band play in every garage in town when I was in high school.
Pink Floyd - Dark Side of the Moon. I'm not a huge fan of psychedelic rock, so their stuff just never really did anything for me.
Slayer - Reign in Blood, Seasons in the Abyss. I can't get into the vocals, the lyrics, and I find the music incredibly over-hyped. I feel the same way about Motorhead.
As with previous posters, I don't get the appeal of U2, The Grateful Dead, or most Judas Priest (never could understand the big deal about Halford's voice, either).
The worst: Bob Marley. Anything by Bob Marley. Anything reggae. Anything with even a slightly "island" vibe. I'll include Jimmy Buffett, who is just sad. I wish we could take the entire reggae/island music genre and sink it into the ocean forever.