Stonemason: The only non-Alchemy card you can buy with a Potion (I'm not sure if you can overpay for Doctor, Masterpiece, or Herald with Potion, but even if so, Stonemason's still the only non-Alchemy card you'd have any reason to buy with a Potion).
Caravan Guard: The only Action card you can play during a Buy phase (in response to Relic).
Gear: The only optional Duration (that is, the only card where you can choose whether it stays out past Clean-up).