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?
Diablo 2. I was still in school, parents had dial up, and it was marketed in such a way that I thought it was mostly catering towards the online experience, so I blew it off. Flash forward nearly 20 years, get into D2:R, single player is actually pretty cool
D2:R is such an interesting technical showpiece. I love that the new graphics are like an interpreted realtime overlay. That the regular game is running right behind all that. Such a cool thing to see. It would be awesome even if it wasn't still fun to play, but it also is.