I used to have a go-to answer for this but I have since been able to experience it again: "City of Heroes".

It was the first MMO I ever reached max level on, the first one I had meaningful social interactions on, a game that inspired imagination in me. It was really special to me in the time that I had it and would always come back to it off and on. And then it shut down which was devastating to me.

Since then though private servers have become available and I tried it again. I liked the nostalgia but realized it hasn't kept up with where I've gone in life. I still treasure it, but now as a fond memory of those specific past experiences and what they meant to me at the time.