(Trying to keep things going on here in the forum haha)
I like the mystery environment, the sensation of being completely alone in there. The Egyptian and Inca/Maya references are also very dear to me, and the many possibilities of exploration. But mostly, the environment it's completely unique.
At first, I used to play the game for that because I wasn't fluent in English and couldn't understand the whole context and read the letters properly hahaha So when I got back years later and started playing it all again, but this time understanding It, It was like discovering a new game, and sub got EVEN better (but way different from what I imagined).
Btw, my fav is LFL, but from the main series, definitely the lab.