I think there is no ranking site that can be 100% trusted.
That said, I trust linux-hardware.org a bit more than distro watch, even if it’s not as popular, because you have to intentionally download an app/script for it to scan and upload your distro/hardware data (so no page clicks or just traffic, you must have the distro installed), and if you repeatedly try to upload the same distro/hardware data, it doesn’t count multiple uploads on its statistics, if they are not at least a month apart.
Edit: and even on linux-hardware you have strange results like OpenMandriva and ROSA as Distros on top 15, and I have never heard of them outside there, and from what I can find they are somewhat popular in Russia and some parts of Europe
As I said on the first line, no ranking of any kind can be trusted 100%, I pointed out an alternative to distrowatch, and why I would trust it a bit more, not saying I really trust it, or that I believe every result.
It is less popular so it could be a case like OpenMandriva has it integrated to upload automatically for all its users by default, or they found another way to game that ranking.
When I see any ranking, I do research when I see a distro that is suspiciously positioned, and I haven’t heard about outside the place I saw it referenced, and even so I always stick to mainline distros.
Honest results would need a standard way that every distro adopts and make an opt-out (not opt-in) regular upload thing similar to what linux-hardware.org does, and be actively trying to mitigate or deny certain distros or specific actors from tampering with the results, and we don’t have that.
Page rankings, clicks, scripts, etc. are not enough if every device doesn’t ping it in a legitimate way (fake user agent or other means), and there is always the case of people that will opt-out or block this as they don’t want to be tracked.
On your point of something like Alexa Page Rankings, the thing I would add is that, at least for me, if it is a ranking shown by a corporation, it is not trustworthy.