I enjoy the base set and every expansion and promo that has been released. I am very excited about the next expansion also. There will never be too many cards for me. However, I have to say that I still have to put Prosperity as my favorite. Maybe it is sentimental. It was the first expansion that I purchased. I got it and the base set as my introduction to Dominion. But my wife and I really enjoy including Platinums and Colonies in our games whenever possible.
We don't mind that it takes a little longer to play with them in the game, though I do have to admit that when someone has an advantage in Colonies, it is a lot harder to overcome than an advantage in Provinces. And some kingdoms are just not a good idea with Colonies. Still, we love it when we can use them. There might be some expansions that have higher percentage of my "favorite cards" in them per total # of cards in the expansion, but Prosperity definitely has some great cards. Who doesn't love Watchtower? Bishop, Goons and Monument add something unique with the Victory point tokens. Mountebank is a dominant attack. And I love City, Grand Market, Hoard, Bank, Peddler, and one of my favorite villages, Worker's Village. (I love having extra Buys)
So here is my list:
1) Prosperity (see above)
2) Dark Ages - Shelters, a $1 card, Ruins, Spoils, and so many great cards, like Squire, Forager, Junk Dealer, Hermit, Market Square, Fortress, Ironmonger, Cultist, Count, Marauder, Rats, etc. I would pick this #1 if it wasn't for that sentimental thing with Prosperity and the total game changing nature of Colonies.
3) Cornucopia - For such a small expansion, so many great and powerful cards, including my favorite alt-victory card, Fairgrounds.
4) Hinterlands - Lots of favorite cards here also: Tunnel, Scheme, Jack-of-all-Trades, Highway, Crossroads, Spice Merchant, Haggler, Border Village, etc.
5) Seaside - Durations, especially Wharf, and lots of fun and/or useful cards
6) Intrigue - My second expansion that I purchased. I love the action/victory cards and how they relate to other cards in this expansion and other expansions. Also, despite its age, it has some of the more popular and/or powerful cards in the game, like Bridge, Courtyard, Masquerade, Ironworks, Torturer, Minion, or Duke. It also has the cards with choices, which I find to be a fun mechanic. However, it has some cards I really dislike, such as Saboteur and Tribute.
7) Guilds - I love the coin mechanic, and there are some fun cards, but overall probably loses points for being a small expansion.
8 Base - I know it is very vanilla, but there are a large number of cards there that still are among the most popular to buy when they show up in a game - Chapel, Cellar, Gardens, Lab, Militia, Smithy, Council Room, Moneylender, Throne Room, Library, and of course, Witch.
9) Alchemy - I like the Potion mechanic, but it is a small set and unless you have several of these in a kingdom, they often aren't worth the effort of buying the Potion. Familiar and Alchemist are definitely strong, and Apprentice is not only a great trasher, it doesn't require Potion to buy it. But I have absolutely no use for Possession.
10) Promo - Good for what they are, specialty cards, but can't compete with the volume of good cards in the expansions and base set.
Thanks for all of the fun, Donald X!