Mine is easy: Cold Lake - Celtic Frost
There aren;t proper words to express how utterly cheated and disappointed I was with this. After the majesty of Into The Pandemonium I was so looking forward to Cold Lake. Its even a great name for a CF album. Then I put it on and.....aaarrrgggh. So awful I've never given it a second listen. I hear Tom refuses to even discuss it these days.
(edited to reflect @MacabreEternal's positive comments about promoting discussion)
JTFI gave a Damn to noblastbeats in Worst album by one of your favourite bands