Also, for what it's worth, if you do average the top 20% this is what you get:
Intrigue: 15.4
Dark Ages: 15.6
Cornucopia: 20
Seaside: 21.2
Hinterlands: 22.6
Prosperity: 23
Base: 36.8
Alchemy: 40
Guilds: 42
Kind of surprised to see Guilds and Alchemy still so low, and Intrigue so high. Intrigue isn't really an expansion I think of when I think of powerful cards. I think some of the Intrigue cards are overrated, mind (like, Steward, is a good card, but I wouldn't say it's top-10 good, much less top-3), but that's just my opinion and WW's played the game a lot more than I have.
In fact since I've made the list, let's also do the bottom 20%. I guess you could call this a vague measure of which expansions have the fewest weak cards... ish?
Base: 198
Intrigue: 192.6
Alchemy: 192
Seaside: 191.8
Prosperity: 178.8
Dark Ages: 173.1
Hinterlands: 169.8
Cornucopia: 164.7
Guilds: 161.7
So while Guilds has the fewest power cards, it also has the fewest weak cards - even it's weak cards are pretty competitive. This could be ascribed at least partially to Guilds newness, mind - it's hard to put cards into the outlier areas until you're pretty experienced. Base, Intrigue, Seaside and Alchemy all have a decent number of low ranked cards, and so have notably lower positions than other expansions.
And since I still have the data, how about a simple measure: the mean position of cards.
Cornucopia: 75.31
Hinterlands: 88.81
Dark Ages: 99.20
Guilds: 99.23
Prosperity: 103.08
Intrigue: 103.96
Seaside: 107.46
Alchemy: 117.67
Base: 121.08
Surprised to see Hinterlands so high up, and Seaside so low. I guess because while Hinterlands doesn't have many power cards, it doesn't have all that many duds, either - it's lowest card is Nomad Camp, a full 28 positions above the lowest position. Seaside on the other hand has lots of meh cards.
And since I
still have the data, how about some bar charts?
I removed promos, then counted (well, my spreadsheet counted) the number of cards in each quarter of the list - so top 50, then 51-100, then 101-150, then 151-200. This chart shows the distribution across expansions. The first chart is unweighted, so Dark Ages is massive on each section (but more massive in Q1 and Q3), while the second weights expansions inversely with size, so that each expansion is in total taking up the same area (hence also each bar having a different height, since e.g. one Alchemy card counts almost three times as much as a Dark Ages card)
Well that was fun!