If we discount stock pickups installed in off-the-shelf guitars, then the worst after-market pickup I've tried was the Duncan Custom bridge humbucker. Perhaps my Gibson V was ill-suited to it, but it was utterly VILE - thin, weak & muddy all at once.
I hated that pickup with a passion & as a result hardly played the guitar for years. It now has a Miracle Man & gets played a lot more :)
EDIT to add: A true measure of "worst" is if the pickup's malign influence stretched beyond the lousy sound it gave out. In the case of the Duncan Custom it did - I held off buying a BKP Cold Sweat bridge for a few years because people on this forum (you know who you are, hang yyour heads in shame!) said it was "like" the Duncan Custom :)