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