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.
You're worrying too much about the specifics.
When I say usage frequency, I just mean it comes down to how often you pick it up.
Dominion is a squinty game, it's okay to squint. With something like Page, that you pick it up every game nearly speaks to the power of Champion and the other stuff. You're not picking up Page for Page, but that doesn't make Page weak by any means.
You are right that Silver is picked up a lot, merely as a stepping stone, but it is still picked up a decent amount of times, and is thus a decent opener. But there are plenty of games where you don't in fact pick up a Silver. The point is, you do have to get a Silver sometimes to get to other cards, and if you do have to use a Silver, hey Silver is decent sometimes. It's better than people say it is, because it
does accelerate your deck. If it doesn't, that means there's a better route, whether that be a Steward, or an Ironmonger, etc. You say you'd never pick up Silver if you didn't have to, and I agree. I also wouldn't use any other trasher besides Donate if I didn't have to. We're arguing all of the boards, not the best case scenarios.
Black Market, who cares. Completely different rule. You're picking up Black Market every time because it can supply something to you that is ordinarily not in the Kingdom, whatever that might be. You don't worry about non-supply cards, those come packaged with whatever card you can in fact purchase. Knights? Yeah, I pick those up a lot. Don't worry about specific Knights, that there are Knights and you often pick them up is good enough!