Noble Brigand, by a country mile. At first I thought "okay, better Thief, maybe I should try this", and lost with it, then I thought "nah this card is still garbage" and started to lose without it, often on the back of some early, lucky, Silver-stealing, and even now I'm trying to get a more nuanced picture of the card and not doing a very good job at that. I mean, often it's irrelevant (and when other players know that too, which is usually, that evens the playing field), and when it's not irrelevant it promotes the sort of game I flail about with. Also I sometimes pick it up as a desperation buy and that doesn't help.
There was a period where almost all of my "worst cards" were from Hinterlands, which is pretty common I suspect: we've had less time to get used to it, it upends/devalues a lot of traditional engine strategies in favor of decks that actually like Copper and Silver, some cards are very very tricky to use even in skilled hands (Develop, Mandarin) while others all too often lead to brainless, luck-heavy rushes (IGG, FG). And the general level of competition on Iso has been steadily improving, so win rates are going to be down across the board.