Only amateurs dream in Tetris. I've dreamt large portions of Quake. It is a truly surreal thing to dream you're IN the Quake world.

By saying that Quake is my choice. It was the first FPS I played with free move and look. No game have captured that Being There feeling quite like that afterwards. There are certainly better atmospheric games but for me that game will always stand out in that regard.

Amateurs...or old people...like myself :)

Definitely System Shock 2.

Definitely the Stalker games, nothing immerses me quite like those.They managed a perfect blend of graphics, sound, art direction and gameplay to really bring about a foreboding semi-post-apoc mood.

Sanitarium.

I think Elmofongo can take it. :P

I'll give you a big +1 for that. Two words I adore "different" and "imagination". You simply can't go wrong with either.

Well, only if you played the original version including the music (by Trent Reznor). I must admit that the first version I played was a pirated copy a friend gave me, then I played the Steam version which also lacks the music by default. Without the music it just felt like some weird 3D experiment to me. Thanks to the Ultimate Patch that some user provided in the Steam forums I finally got a chance to play it with the music two years ago or so and only then Quake turned into that incredibly atmospheric surreal experience - it totally blew my mind.

Anarchy Online.

It's not much to look at anymore, but the variety of settings, the surrealism and the music combined for a vibe unlike any other game.

Pardon the music in the video. Here is what the real music sounds like.

Absolutely. The first chapter (in the town, not the weird prologue) was some of the most powerful and memorable stuff I have ever seen and heard in a game, maybe mostly thanks to the brilliant soundtrack.

Just FYI, Quake IV comes with the original, and while it is the same graphics they tweeked it quite a bit to make it look really nice on newer systems. I'm not big on the music but they (and you know they) did a good job on the lighting.

I know this is probably blasphemy and I'm quite alone in my view, but I was actually largely disappointed by Quake. Maybe just because after all the hype, it couldn't possibly live up to what I had pictured in my head. Real 3D, soundtrack by Reznor, dark and moody? OKAY SIGN ME UP! Oh wait, most of the levels looked and felt the same to me, everything in brown and darker colors. Maybe I was just used to bright, colorful, "in your face" games like Duke, then having Unreal come out and not only be pretty, but wide open.

Best is such a strong word. I'll probably come up with different answers depending on the day of the week.

But one that was really good was Demon's Souls. Despite being able to see other players it still had a very lonely and oppressing atmosphere, like the whole world was out to get you, and it was.

I just bought Sanitarium from here yesterday and have only played it for a few minutes but I totally agree with regards to the music. Also, am I the only one that finds that child's 'echo' voice in the main menu kind of disturbing?