That's one of the things I hoped to address in this thread. I too have a new favorite every week/month/day/hour. But (for me, at least) there's always been one game that sticks out.
Viewing post in What is your favorite game?
I did say until this year though.
I've changed how I look at my favorites now, such that at least I can nail down the top 10.
Which ones had the most profound impact on me in some way? since not many manage that.
And of course as I said before, Eternal Senia blew me away, so it was obviously #1, but that was only this year.
Maybe a thread on "what really sticks with me from games." might put a light on the matter.