The exact answer is clearly Curse, but there's another question of cards that might in certain circumstances be attractive buys only with an Embargo token.
Fairgrounds is a possibility. You typically have 10 Kingdom cards, 3 VP, 3 treasure, and Curse. If there's no +buy or other cursers and either Horn of Plenty or Feast are on the table (that is, you have a potential expectation that you will not have any of a particular Kingdom card because it provides more points trashing it), it would be possible to be interested in Fairgrounds only when it's Embargo'd.
Maybe the Ambassador pile could fit too. If there's no +buy or other cursers and you opponent doesn't open Ambassador and you open with just one, it might be reasonable that you're more drawn to buying a second when you're going to get a curse in your deck for free.
Menagerie with Secret Chamber or Vault, or maybe even Cellar, on the board is a bit of a stretch, but you might think that one more Menagerie would be a good idea if you can get another unique card in your deck to make it more likely to fire off big Menagerie draws which turn into cash or other cards.