Thanks, for me it's been a fairly active process over 40 years of watching old movies and intentionally seeking out and watching things I know are considered classic by most, but I haven't seen yet, that's not that often anymore but I still find new gems to watch. Going with a genre, like noir, or thrillers, or early comedies, or silent films, or a specific actor like Spencer Tracey mentioned above and searching out the best of those sometimes turns up a new masterpiece once in a while or just a really good movie or performance. I still remember the first time I watched Bad Day at Black Rock with Spencer, Robert Ryan, Borgnine, Lee Marvin, Dean Jagger etc ... it was great.
I agree with your earlier comment about fav films is an individual thing, even though there are hundreds that most people will agree are true iconic classics if they'e taken the time to watch them. But I could never come up with a definitive personal top 100 .. there are just too many great movies. I could certainly not come up with a top 10, or rank order my favorite movies and I don't see a point personally. There are just movies I consider to be a personal classic and will, and do, watch over and over again ... that's my personal distinction.
If I'm channel flicking and come across one I've seen before but cannot change the channel even if it's in the middle or near the end. That's what I call a top caliber classic movie.