For a long time, Gardens was the hands-down answer here. There's a reason I have more Gardener achievements than anyone else. Then Hinterlands came out and Silk Road was newer, shinier, better. I've since cooled on both of them to some extent, so they're probably not my favourites anymore. But I still like them. Anyway, I give you a list of my favourite cards, along with some that I just like a lot more than an objective measure (or other people) would.
Monument. Man, you can do so much with this really simple card. Those VP chips do a LOT for you. Of course, bishop tends t outshine it in a lot of situations, but for a good while now, I've preferred monument. It's good in BM+it+cantrips, and it's good in lots of engines, 'cause it gives you a chance to catch up.
Workshop. Sure, it gets outclassed by ironworks like warehouse does by cellar, and yes I like Ironworks more, but lots of people like ironworks, and nobody likes workshop. And it's not just SR and Gardens rushes, people. It's great for all those engines where at least a couple of your key components (and, most villages do) cost $4 or less.
Ironworks. See above.
Horn of Plenty. I've never misevaluated a card worse than this one. Well, maybe saboteur in the very very 1st, 6-player game of dominion I ever played. But that was 1 game. This, this I thought was going to be all kinds of terrible when Cornucopia came out. But not only can it play non-terminal ironworks for you (hey, it at least gets a silver most of the time), it's really nice in lots of engines and LOTS of cases where you can go mega-turn. No other card lets you realize how many cantrips are out there. Or, in general, makes spy so useful.
Courtyard. It's so nice in big money, and if you've got an engine, it's really nice there too (just not SO nice as the ONLY card-draw).
Governor. Not because I think 'woo, governor' every time it comes up. But because I think like really NOBODY I've played/talked to understands it real well. Maybe Donald does...
Merchant Ship. If you read the secret histories, you see that they added +buy to wharf to make it better in comparison to this card! A very underrated card, I think, its very often a better source of coins than gold. It works pretty well in BM land, and in engines, too. The duration thing really helps it out. Well, I'd wax more on this but... I think I'm going to write an article on it soon....
Steward. You know, I think this card isn't super-well understood either. It's pretty darn strong. The flexibility gives you good options pretty much all around. It's a GREAT card in engines, and really amazing with KC. Oh, ok, that's true of most actions...
Throne Room. Whereas KC feels degenerate, this card feels pretty natural and fun to work in as just another cog in your engines. It's so rare to have a card that a lot of times, you're like 'ooh, gimme silver' or at least 'why do I want this over the other, more pressing components?' but others, you're like 'forget grand market, I need more throne rooms!'
Margrave. Just because people don't understand that you typically don't want more than one or two of these in your engines, if you can use say smithies instead. The attacks anti-stack, people!
Mandarin. Yup, love this one a lot, too. Yes, it anti-cycles you, and yes, terminal gold is not so flashy for a 5-cost (no really... think of how many 5-costs you want more than non-terminal gold... and how many of them are also terminal), BUT it can play a really nice role in lots of situations. And it has some collision prevention (I had to type that like 3 times for it to not come out collision detection...) built-in. And the buy effect (I guess it's a gain effect, isn't it?) can be pretty darned useful as well.
Haggler. Another hard-to-evaluate card. Can be really good when there's lots of stuff you want at different price-points. Which looks to say engine. But it also lets BM not lose steam by playing a kind of hoard for you.
Lighthouse. I never understand how people love fishing villages so much on boards where they have no terminals, (or one terminal and it's non-draw), and let are so meh on this card, even when there's attacks.
Moat. You know, blocking some attacks is pretty important. And in an engine, draw two cards isn't THAT bad... particularly when those curses would bust you, or when you have a zillion extra actions floating around.
Herbalist. You know, that treasure-topdeck is pretty nice fairly often. Even when it isn't, you need a +buy? This isn't the worst you could do.
Wharf. What can I say? It's the best card for big money (oh yeah, cursers
) in the game, and one of if not the best card for engines. It just does everything you want. Everyone knows it's ridiculously powerful, and yet... I STILL think it's underrated. Heck, you know how people will somehow list cards that are single-card engines (minion is most notable)? THIS is basically a single-card engine. BM Wharf Actually plays almost like an engine - it blurs the line between engine and standard BM, at any rate - all in a single card. It can make coppersmith good, it makes outpost potentially quite good, it gets you to think 'man, I'm overdrawing my deck here... should I grab some more coppers?' And it's so adaptable. Your opponent building up a bit? Fine, you can play that game. 8 is now wharf+silver (or wharf+village) instead of a province. They preparing to lightning rush some stuff? I can dip into provinces sooner than normal to counter that too. And my deck won't be totally crippled for it.