Base: Chapel/Feast
Intrigue: Bridge/Shanty Town
Seaside: Wharf/Embargo
Prosperity: Quarry (tied with King's Court)/Trade Route
Alchemy: Scrying Pool/Familiar
Cornucopia: Horn of Plenty/Harvest (I actually kinda like tournament).
Hinterlands: Scheme/Ill-Gotten Gains
Dark Ages: Band of Misfits/nothing (in a 35 card set. Dark Ages is awesome)
Guilds: never played, but Stonecutter and Herald look insanely fun.
As for the expansions in general, that's tricky. I mainly play IRL, and the sets I like best aren't necessarily the ones my family members like best.
9. Intrigue. Most "dud" cards of any set. Great cards for every strategy. Ironworks for rushes, bridge for megaturns, Duke for slogs, and courtyard for Big Money. But there's also a lot of cards that I just don't get, which seldom contribute to a good deck. I've already complained about shanty town, and of course secret chamber and scout are deservedly mocked. So yeah, a lot of nonexciting cards.
8. Base. Oddly enough, the likely favorite of my brother, who has a soft spot for quiet, situational cards like Bureaucrat or Adventurer. Some cards are staples of dominion. The first village, the first drawer, the first market, etc. Witch and Militia are still excellent attacks, and chapel is an all-time favorite. Still, a lot of the crazy explosiveness of Dominion is missing. In base, a "megaturn" is 2 provinces, and the closest thing to a legitimate "combo" is Council Room/Militia. Not to mention, Feast isn't super interesting, and Thief is a joke as always. Aside from the cards that went on to have variations, like lab and village and witch, the rest is pretty unimpressive. Still, it has chapel, which is probably my favorite card.
7. Guilds. May be great, and some of the cards look like they could be favorites, but I have no right to rank it highly until I actually play it.
6. Alchemy. As an engine junkie, this ought to be my favorite expansion. The odd thing is, while some cards here are fantastic engine enablers, others have a lot of prohibitive tricks here and there. Golem's too expensive, Alchemist is slow, etc. University, oddly enough, often creates a situation where you don't have enough actions to play your own terminals, especially if there are no good non-terminal actions to pick up on university plays. And familiar, awful familiar, adds far too much luck to the all-important curse war. That said, Scrying Pool is the best engine drawer of all time. When alchemy cards shine, they REALLY shine, but you always have to do a little juggling to fit them in.
5. Hinterlands. Actually a very nice set. The "Power cards" of the set, JoaT and IGG, force BM and slog strategies a little to heavily to be my favorites. However, when I look back over the list, it has some real gems. Fool's Gold is a great alt-treasure, Scheme is one of the most delightful engine cards out there, and the on-gain effects from Border Village and Haggler have created some really fun plays. Dominion would really miss something without Hinterlands.
4. Seaside. Who doesn't like Seaside? Sea Hag is a frustrating curser, Lookout is risky, and I'm never happy to see embargo on board. Aside from that, Seaside is fantastic. The theme is implemented solidly, and fans of explosive dominion strategies tend to be nuts over Wharf and Fishing Village. In all honesty, I think Seaside is the "power" expansion of dominion. Tactician is awesome, Island is unique, and even Treasury has a lot going for it. Ambassador is pretty surprising to people who've only seen base, because in a sense it combines the abilities of Base's two power cards, Witch and Chapel. It junks your opponent and trashes for you-so cool.
3. Dark Ages. This is the most mixed set for me. The combos in here are amazing. It's easily the best expansion for a creative engine builder. However, if your opponents go for simpler strategies, or don't quite get the new cards, they are going to be bored out of their minds. I once had a Knights-Rogues game where I trashed and gained half the contents of everyone else's deck every turn, and neither of them had fun. After a while, neither did I. That's the reason Dark Ages isn't my number one: it creates exactly the kind of boards I like to play, but creates more tension at the table than anything else.
2. Cornucopia. Almost as nuts as Seaside, but without any cards I really hate. The theme is beautiful, and carries a lot of favorites for me. Horn of Plenty is great, and I think Jester might be my favorite attack, especially in Scrying Pool games. It's so rare to get an attack that benefits you more than it hurts your opponent, and Jester's mechanic makes it one of my favorite gainers. It's got good cards in a lot of diverse areas, as well. Good trashing, good villages, good draw, good buy, and one of the most strategically rich cursers in the game. I even like tournament: sometimes, you just want a peddler for $4.
1. Prosperity. Not much of a surprise. I'm a megaturn junkie, and Prosperity was made for such people. Everyone remembers the 6+ staples like Grand Market, Goons, and the legendary King's Court. However, I think some of my favorites in the set are Peddler and Quarry. In any engine with easily added +buy sources like market or worker's village, superfluous buys are quite commonplace. Peddler and Quarry are great cards for giving +buy more oomph, albeit in very different ways. Quarry's kinda funny. It won't make an engine by itself, but if one is already viable, it's exactly what you need. I remember one game with Worker's Village, Wharf, and Quarry that must've ended by turn 8 or something like that. Most importantly, Prosperity is fun to lose, as well as to win. When my brother ground my under with King's Courted Mountebanks in the first game where both were present, I knew I had lost. But then he started looking for ways to use KC on virtual money--and the best source was Mountebank. When the game ended, he had literally emptied the contents of the copper pile into my deck. I wasn't pissed at all-I was laughing. That stuff just doesn't happen outside of Prosperity.