There are probably lot of cards that could fit. But I will mention a few very different ones. Also, it's worth mentionning that well designed cards depends on the other dominion cards. For example, I think silk road is a well designed card but could make some games boring due to a workshop/silk road or IW/silk road combo.
Duke : For the game-changing criterion. It completely change the balance of the cards, and still you'll rarely play an annoying game because a Duke/something is too powerful and uninteresting. From all the alt-vp card, it's in my opinion the most interesting (yet not my favourite !)
Develop : So fun and so hard to play at the same time. It adds very cool tactical plays sometimes.
Embargo : Just for the mechanic and simplicity. I mean, it's not the most balanced card of the world, it can depends a lot on luck, but still it adds so much to the game.
Overall, I think the best designed cards are the trashing cards. All are so interesting in their own ways. And I have a particular love for Menagerie, Island, Farmland, Horn of plenty, Baron and Coppersmith…