I'm curious to hear what people consider their favorite cards. Use whatever criteria you'd like. But basically what I'm looking for is the cards that make you most excited to see pop up in a newly-composed game. It could be because they enable awesome turns, because they are interesting and unique in how they work, because they force games in the direction that you most prefer (i.e. - engine folks almost always hope to see a village), or even just because you happen to be very good at playing that card.
I bet that a lot of the favorites are among the most-hated cards for others. King's Court, Goons, Possession, etc. Some people love them and others loathe them.
I answered this question as part of the DS tournament 6 months ago, but my opinions have changed a bit since then, so I'll list five here:
Ambassador - It's an evil, evil card. And it is incredibly frustrating to get on the wrong side of it. But I really enjoy seeing it these days because it means the game is very likely going to reward some serious strategic thinking, not just good tactical play.
Watchtower - It's often a fairly dead card, but I LOVE games when Watchtower has a chance to shine. Watchtower-driven engines are enormously fun once they get going. It really only works when you need both features of the card. You use the draw power to keep working through your deck, and then you top-deck all the key components to ensure the next turn works just as well. And the best part is, you're quite resilient to most attacks. With Watchtower in hand, handsize reducers often HELP you and cursing attacks are meaningless.
This game was particularly fun:
http://councilroom.com/game?game_id=game-20120524-104340-347e6fbf.htmlMenagerie - This is one of those cards that frustrates me a fair amount (when my opponent isn't building around it but just happens to luck into perfect hands), but in spite of that it's a really fun card. It's obviously very powerful, but there's just a lot of fun in building the incredibly diverse deck. Some of my favorite games ever are Menagerie-Black Market ones. You can just go completely nuts.
Fishing Village - This one is pretty obvious. Some decks will prefer to have Worker's Village for the +buy, or lack in +cards from other sources and so don't want to lose hand-size while playing villages. But if you want a village in the kingdom, more often than not you'd prefer it be this one.
Apprentice - Lets you trash Estates early, silver in the mid-game, and Provinces at the end. And all kinds of other things. I like the way Apprentice-heavy games can putter along for a bit and then simply explode.
Honorable mentions: Vineyard, Apothecary, Wharf, Horn of Plenty, Scheme
