Well . . .
The Ma'at comes to mind. Cost a lot of money, and I really despise most of the cards. I bought it because I'm a collector, and that's what collectors do -- buy stuff

. But I really, honestly, should have let that one pass.
Also kind of hate the Mystic Faerie. Also the Bright Idea Deck. Shudder. Bought it on some kind of clearance once upon a time, but seriously, even at under $10, it was a waste of good money.
Oh yeah . . . the eVoluti. Again, it's a collector's sort of deck, and I collect decks, so I bought it. But it was a mistake. Should have stayed well clear of it.