It really comes down to usage frequency.
By 'frequency', do you mean 'at least once', or actual frequency? I might only use (i.e. play) Page once in a game, but it's still a very strong card. I might only use Chapel 2 or 3 times in a game, but it's very strong.
And by 'usage' do you mean 'play', or 'gain', or what? I may gain IGGs, without ever really needing to "use" them, or even wanting them in my deck altogether. And there's gaining by choice (Witch) vs. gaining not by choice (Curse) -- do you mean the former?
There are still flaws with this though. I use Silver a lot -- but often just as a stepping stone to get to other things. Oftentimes I wouldn't use it if I didn't have to. I use Potion a lot in Scrying Pool games. I use Gladiator to get to Fortune - even if I don't necessarily want the Gladiator. For that matter, are split piles by definition top half stronger than bottom half, cus (barring edge cases) it's guaranteed that there are more games in which a card from the top half is gained but not the bottom half than there are vice versa?
How would you do non supply cards? Tournament (and Province?) is strong because of Followers and Trusty Steed, but how would you handle the Prizes?
How would you do Knights/Ruins/Castles?
How does this definition handle cards from the Black Market deck? Rats is (arguably) stronger if it came from the BM deck; Magpie is weaker. Why?
If you do mean something like 'percentage of games this card is in in which you gain the card by choice at least once in the game' then, off the top of my head, Butcher comes to mind (Dungeon in your case). There are probably fewer games that I ignore Butcher than there are games where I ignore Cultist and Monty. But I'd still say Cultist and Monty are "stronger" -- air quotes, because as this thread suggests, I don't
really know what makes a card strong.
