Prosperity is I think the only choice for #1. You really have to give the good cards more weight than the bad cards in an analysis of this type, and Prosperity is the obvious choice for biggest and baddest expansion. KC, Goons, Grand Market, Mountebank, Peddler, Hoard are all among the game's absolute killer cards, and even most of the weaker Prosperity options can be pretty good- only Counting House and Talisman are bottom-tier IMO, even add Contraband in and Prosperity is very light on the bad cards.
My #2 vote is
Cornucopia. Yes, Cornucopia above Seaside, by a wide margin in fact. What Cornucopia has going for it is not just a high proportion of elite cards, but a total lack of bottom-tier options- every card here is pulling its weight, leading to what might even be a higher "batting average" than Prosperity. Hamlet, Menagerie, Tournament, Remake, and Hunting Party are all uber-elite at their price point, and Young Witch is almost as good- fully half the set is power cards with a capital P.
#3 is
Seaside, which IIRC Theory once called the only expansion that was actually power-creepy. It's a heck of a lot better than the first two, but I think there's more space above it than below it now. Fishing Village, Ambassador, Sea Hag, Wharf and Tactican are the only cards I'd call truly top-tier here, and there are more bad cards (PS, Treasure Map, Explorer, Smugglers). But there are also a bunch of near-great cards that put its average well above what came before, like Caravan, Lighthouse, Warehouse, Salvager, and Ghost Ship.
#4 is I guess
Hinterlands, edging out Alchemy by a nose. Hinterlands is somewhat hard to figure, since there are very few true must-buys (JoaT and IGG are probably the best of the bunch) and more than a few ultra-niche bad cards (Duchess, Cache, Mandarin, Develop, Noble Brigand). But, the meat of each set tends to be its $5s, and most of the $5s Hinterlands has are pulling their weight- Margrave, Embassy, Stables especially. Add in a couple more strong options at each end (Border Village, Tunnel, and sadly Fool's Gold probably belongs here too) and the average is pretty strong.
#5 is, as mentioned,
Alchemy. Not a lot of cards, and three of them are pretty bad (Transmute/Herbalist/PStone), but each other card here can frequently dominate boards, since it has to be good enough to justify Potion buys by itself. Apprentice and Familiar are clearly the best of the bunch, but Scrying Pool and Vineyard probably also deserve to be called elite.
#6 is
Intrigue. IMO, only three of the cards here would be better than average in Cornucopia- Masq, Torturer, and Minion. A couple others come close: Nobles and Courtyard. It's better than the base set, but that's not saying much.
#7 has got to be
Base. When they made the Base set simple, they necessarily made it not very good. It says a lot that Smithy-BM is strong in base-only environments and is pretty damn bad everywhere else. Three top-tier cards (Chapel, Lab, Witch), and a huge gulf after that. The kicker is that base set has more really bad cards than Intrigue, including the worst card in the game.
#8 are the
Promos Governor's good, but there are no must-buy promos, and the other four are mostly lackluster.
...
There was a definite jump in power level from the two base sets to the more complex and powerful expansions, but I don't see a clear pattern since then.