Oh, this is easy...... The much-dreaded ART Nightbass. That damn thing gave me fits!!! I delivered newspapers, pizzas, cut grass, played gigs, literally everything to get one of those. I finally got myself one, and that damned thing..... Man, it sounded like crap. So friggin' cheezy-sounding. I was PISSED, to say the least. I think that's why I'm more of a purist, when it comes to effects.
I can't think of another piece of gear that made more people sound like crap. The main amusing thing was how many folks I know who changed everything else about their gear because they didn't want to consider that the ART was the culprit.