I definitely like Embargo as a counter to Potion. It definitely hurts Scrying Pool, and does quite a bit of damage to Vineyards and University, probably. And it makes Golem, Apothecary, and Possession somewhat less palatable.
But if you could only list one, Alchemist is the card that is utterly crippled by Embargo. It will occasionally happen where someone will see Alchemist and open with Potion, but not notice Embargo, since it's sort of an innocuous $2 card, often skippable. Oh, how it hurts Alchemist. Alchemist isn't so great anyway, but it's wildly misunderstood, and people go for it baffling often.