When I first started using password managers, I tried a LOT of them (more than 20, maybe as many as 30 of them). One of the criteria I set was that of the about 200 logins I had, it should not fail to fill on more than 6 sites. My reasoning was that to prevent phishing, it should always fill. A failure to fill should indicate a fake site. If it fails to fill on a valid site, then it is not providing the phishing protection that it should be providing.
Bitwarden fills all sites (though some needed additional setup to work). KeepassXC fills all sites (with more special setups than Bitwarden). Proton pass when it was first released failed on 32 sites. Now it fails on 11 sites. Proton Pass does not provide a way of tweaking the fill to fix the “problem sites”. It also does not support sites with anything other than a username + password format, so things like 3 field sites are not supported. It also only appears to match the base domain of the site, so suggests multiple logins for sites with different subdomains, though I have not investigated this further. 1Password when I tried it some years ago also has issues with subdomains (matching just the base domain like Proton Pass).
As for UI, 1Password is the nicest one, though like all the others it has some less obvious things. Original Bitwarden was plain but highly functional, the newer UI is prettier but not as quick to use (in my opinion of course). Proton Pass is prettier than Bitwarden, but for usability I would rate it worse than 1Password and Bitwarden original. Also the integration with Simple Login has traps, like making it easy to delete an alias which can then never be recovered. I run Proton Pass unlinked from Simple Login for this reason.
As others have stated the 1Password Secret Key is a left over from the days 1Password was a local password manager. All it does is similar to adding some extra characters to your password so if you use a weak password it will not be too weak. If you know how to create and use a complex password it is more hindrance than benefit as it is another way to get locked out of your passwords.
All of these better known password managers are evolving so my comments apply to the versions available now for Bitwarden and Proton Pass, and to a few years ago for 1Password.