Mine are Half Life and Morrowind.
I feel like back when I didn't have a massive game library I would have spent so much time replaying Half Life and exploring every inch of the maps and trying to find exploits to break the game. The massive modding community likely would have kept me playing for years.
Morrowind probably would have gotten me a lot more into RPGs and fantasy games. I do love RPGs but still struggle to immerse myself in fantasy settings. I assume this is because I mostly played SciFi games growing up.
What about you? Is there an MMO or something you maybe tried and gave up on to soon?
There is a bunch of different modern versions of Myst. It's also got a VR version that is very good. Riven and Obduction are also available in VR. Not sure about some of the lesser known Myst games like exile, uru, or revelation.
In my experience, playing them when I was younger didn't work out great, some of the puzzles were just way too hard for pre-teen me. But they were great to play now.