What's your favorite Adventures card(s)? What about Empires?
Base: Throne Room
Intrigue: Pawn
Seaside: Smugglers
Alchemy: Apprentice; for cards with P in the cost, Golem
Prosperity: Peddler
Cornucopia: Menagerie, though Tournament is close, Tournament-haters
Hinterlands: Develop
Dark Ages: Rats
Guilds: Herald
Promos: Black Market
Base Cards: Copper
Adventures: It seems unfair to pick Page/Peasant, since they're each 5 cards, but man I pick those. I'll give Peasant the edge because I don't have to think "maybe people hate this because Warrior could kill their Warrior." Aside from those, Transmogrify. For Events, Inheritance.
Empires: It's a harder call here due to the lack of any kind of test of time. For regular cards, just trying to think, what will I be happy to see on a board today, I am picking Enchantress and Catapult, though that's not exactly fair for Enchantress because it's one of the last cards added and so has some new-card love affair going on there. Villa, Temple, and Settlers are all stand-outs too. For Events, Banquet. For Landmarks, uh man. Wolf Den, Arena, Battlefield, Fountain.
For the expansion question, if I group them into tiers and just consider the big expansions (it's harder to fairly compare the small ones), the low tier is Dominion and Intrigue, next is Seaside, next is Prosperity and Hinterlands, top is Dark Ages, Adventures, Empires. That's unfair too because the last three are all extra large in some sense. The thing is though, that if you are in a Business, and interact with people in what might be considered a Promotional way, then the only acceptable answer is, that the most recent thing is the best thing. Now as it happens the later stuff tends to be better for some kinds of things, because you get better as you go along. And that has certainly happened with Dominion (insufficiently countered by worsening due to complexity or using up good ideas). But I don't want my answer to be suspect, is he just pushing the product or what. And I don't want to be that awful guy, from a publisher's perspective, that guy letting you down on promotion; man the Ascora Games guy was Not Pleased when someone in a BGG thread asked me to pick which was best from a subset of my games, and the answer wasn't Nefarious (I put Nefarious pretty high up, but I prefer Dominion, and don't feel like that's unreasonable).
So anyway I mean. When you (not you mail-mi) ask a question that to me amounts to "which publisher do you want to be happiest" or "do you prefer being honest to your fans vs. having publishers want to publish your games," well you know, I prefer being honest, but I think I can just not answer some of those questions, I think that's the move there. Which expansions do the fans like the best, that's a way better question.
It is also tricky between the three because Empires is newer and so fresher which raises it, while Dark Ages is much older but gets some nostalgia boosting it there, and Adventures has the negative association with people being pissed about the bendier cards.