Mine is ‘Anchorhead’ (1998), by Michael Gentry. I think it’s actually my favourite game of all time, of all genres.

I’ve played the old, text-only, Z-code version back in high school, around 1999, and the experience was so vivid and immersive that to this day I can draw a map of Anchorhead from memory and recite the lineage of the Verlac family. I think it’s still my favourite game of all time (although I spent much more time on some others).

These days, an illustrated version can be bought on Steam for something like $10. Highly recommended!

...which you can also play online: https://ifdb.org/viewgame?id=op0uw1gn1tjqmjt7