Old school runescape. I played it as a kid and have never otherwise had such a wonderful and memorable experience playing a video game. Im quite young (23) so when I was playing it back in 2006-2008, I learned a LOT about life. Trading and bartering, getting scammed, talking to people to get help moving forward in the game, etc. Most of my friends who used to play agree.
You can still play, they brought it back a few years ago, but the community and popularity - as well as the feeling of discovery I used to get while playing - won’t ever come back