The first ever game I bought on Steam. I finally played it to completion, 13 years later. Having received near constant updates during that time period, it was so choc-ful of content that it took over 100 hours to complete.

Every minute was FUN though, and I had sort've done just-barely-enough to complete the game. There is still loads of content here if I want it.

A serious contender of a game for me, in one of those "If you could only play one game for the rest of your life" hypotheticals.