This is so difficult. There aren't any I dislike and it's really hard to compare expansions of other sizes; in larger sets it's just more likely there will be cards I love. Guess I'm just considering themes/mechanics, not the specific sets of cards
I don't want to play every game with Platinum/Colony, so I didn't vote for Prosperity.
I love Cornucopia because it doesn't just add a new mechanic but encourages you to do something you usually don't: pursue variety. For me, this is a more interesting different experience than Platinum/Colony. The variety will play differently in every Kingdom while Platinum/Colony will more or less have the same impact for me every time. Plus there's some sweet cards, and I don't mean Tournament, there's more there than that.
Trashing also feels so fundamental to me that I couldn't live without it, and Dark Ages obv brings that for me. I like attacking, so Knights are cool. Shelters are interesting. I usually go for Rats if it's reasonable. Plus the combos are fun to pull off. Rebuild, you say? Well, no expansion's perfect.
And then there's Intrigue. Maybe not as fine-tuned as later expansions, but it opened the door for what the Dominion expansions would do. Choices, multi-type cards, and possibly my favorite card (Masquerade). Probably not my very favorite expansion, but it's what got me hooked and it's probably in the top.
Hinterlands is solid and man, everyone knows it. No need to wax on here. I like the actions subtheme of Alchemy, but it usually results in megaturns, which I don't like because it leaves long stretches of time when you're opponents aren't having much fun. (Apprentice is notable, and wants to be in Hinterlands again anyway.) Durations are neat, but I gotta draw the line somewhere. I wish Durations had been used here and there in later expansions; I just never thought the rules length was a reason to miss out on more good ideas and I guess Adventures got over that. Coin tokens and overpaying, well, I guess they never felt like a strong theme on their own. Oh, and Base. Doesn't really have a mechanic, the theme's just simple. And simple is never bad.
Don't know who cares about my opinions and wow this went on too long, but I guess that's what this thread's for.