Voted Transmute.
All of these cards are pretty bad, although Contraband and Explorer aren't quite as bad. And as generally useless as Duchess, Secret Chamber, and Moat are, they are so cheap, that I just don't think they're really capable of doing enough damage to your deck. Not like, say, buying Adventurer over Gold.
Adventurer and Scout for me are close to being the worst, simply because they are so wrongly priced. I love what Adventurer does, but its cost makes it almost never worthwhile. I mean, if there is any other $6+ card available, you are probably not buying Adventurer. And Gold is always available! Scout, on the other hand, hasn't ever seemed worthwhile even in its best case scenario decks--Nobles, Harems, Crossroads, Great Halls. It's better then, but not nearly as important as those other cards.
But at the end of the day, Transmute is the most ill-priced card in the game, thanks to its Potion cost. The setup where Transmute is the dominant strategy, or even among the dominant strategies, has to be rarest among all these terrible cards.