Excel and Onedrive. Doesn’t cost much for a license or O365 subscription, infinitely customizable, store on your own storage, no outages or chance that a company stops supporting it. My problem with all the ones that exist like GameEye is they don’t account for homebrews and adding an entry for every one I buy is cumbersome.
Google sheets is a close second but I don’t like how it doesn’t support tables for sorting A-Z, unless it was recently added. Still don’t like its reliance on an internet connection.
It is really just personal preference though for whatever you use. I was keeping track in Game Value Now for a while as well just for fun to see a value of my collection but I stopped doing that a couple months ago. Remembering to add in two spots was annoying.