Korematsu v. United States which basically upheld FDR's executive order is probably one of the least defensible SCOTUS decisions of all time and definitely one of the worst if you include a few pre-Civil War cases.
ADDED: And, yes, SCOTUS is political too but it should act as at least some degree of check. I'm also not specifically commenting on any particular decision but I've certainly read decisions (as a non-lawyer) that were counter to my personal preference but which seemed the right thing legally.