Most atmospheric game

[Eyeball]

Anvil of Dawn.

 

[Astral Rag]

My favorites:

Thief 1 and 2
SS games
STALKER

Quake
Unreal
Blood

AitD
Bioforge
Nocturne

Dungeon Keeper
Syndicate Wars
Sacrifice

MDK

Fallout 1 and 2
Planescape: Torment
Vampire: The Masquerade - Bloodlines

UFO: Enemy Unknown
DEFCON
Incubation

Interstate 76

I Have No Mouth And I Must Scream
Full Throttle
Grim Fandango
Blade Runner

 

[skacky]

Thief: The Dark Project
Alien: Isolation
Arx Fatalis
S.T.A.L.K.E.R.: Shadow of Chernobyl / Call of Pripyat
Ultima Underworld: The Stygian Abyss
System Shock 2
Quake 1
Unreal
Silent Hill 1 and 2
Aliens vs Predator 99
Doom 64
Metro 2033 and Last Light (would've been even higher if they didn't have these cinematic moments).

 

[Unkillable Cat]

Ultima Underworld: The Stygian Abyss

BRB, kicking myself for forgetting this one.

 

[Raapys]

Archimedean Dynasty
Battlezone 1
Inquisitor
Witchaven
Uplink
Alien Legacy
Heroes of Might and Magic 1-3
Master of Orion series, particularly the second

 

[UndetectedButthurt]

The first Max Payne is so good that I replay it every winter, it doesn't take itself very seriously but it really plays like an old action movie.

Oh man... Running around all those druggie infested back alleys in the first few levels... Gud stuff. The extreme-winter-conditions-in-new-york thing was done really well. Back then Remedy were bros...

 

[Astral Rag]

Gabe has all the answers.

 

[rogozhin]

Diablo 1 - Audio-visual experience of this game is just amazing, I wish more RPGs tried to imitate it's dark gothic atmosphere instead of going rainbow colored WOW crap and boring high fantasy with hippie elves and Scottish dwarves.

Yeah, Diablo 1 was grimdark done right. The subdued colours, sparse music and creepy sound design worked just perfectly together. Blood Omen: Legacy of Kain was also amazing in that regard.

Other than that, Primordia cannot be mentioned often enough, and Blackwell Unbound did a great job as well, especially sound-wise (I totally fell in love with Lauren's voice).

 

[Gentle Player]

Ultima Underworld
Unreal
Thief 1 + 2
System Shock 1 (didn't like 2 as much. SHODAN felt like a glorified quest compass rather than the menace it was in 1. Plus farcical endgame and ending)
Myth
Homeworld
Age of Wonders 1
Creatures
King's Field (particularly 4. Dat music)
Blood
Abe's Oddysee
Dark Souls
SMT Nocturne
The Last Express
The Void

 

[Gersen]

Dark Earth comes to mind first. So much work went into creating this world, a shame it's been long forgotten.
Ecstatica, especially the first game that plays like a weird dream.
Sanitarium
Mechwarrior 2: this one may be more subjective, but the way the soundtrack fits with the slow-paced gameplay creates something unique.

 

[Spectacle]

In addition to many good games listed so far, I want to mention GTA Vice City. The music, the clothes, the cars, it all comes together to make you feel like you're in an 80s crime movie.

 

[SCO]

Azrael's Tear for me suckas

Some fan missions of Thief surpass it ofc, but not on backstory obtuseness, which is the coolest part of immersion if you're not a total idiot. Dat feel when you suspect there is something hidden and there is and the designers didn't push it right in your face.

Many excellent games mentioned here too, already being forgotten on the wider web.

 

[Goat Vomit]

Blade Runner.

 

[Kouvo]

Disciples 2, especially the soundtrack.

 

[MasPingon]

Outcast, Grim Fandango, Left 4 dead, Half Life, Dark Souls, Sanitarium, Max Payne 1, Witcher 1, Stalker: Misery, Another World, Chrono Trigger.

 

[BelisariuS.F]

Outside of rpgs: Legacy of Kain series, Max Payne, Medieval Total War, Trine 2 (and Clive Barker's Undying until I've grown immune to horror games). But the single most atmospheric game for me is The Witcher 1 (art + music hits all the right spots in my brain).

 

[SirSingAlot]

Baldurs Gate 1, not close

 

[Phage]

From what I've played... (mind you I've got a 70+ game backlog)
The obvious codex choices:
FO1, Planescape, SS2, Morrowind, Dark/Demon's Souls


Moving beyond the codex obvious:
The Dream Machine, Metroid Prime, Yume Nikki, Castlevania SotN, Hotline Miami, Bastion/Transistor


Some other mentions depending on your definition of atmosphere:
Papers Please, The Stanley Parable, Alpha Centurai, Banjo Kazooie, Ghost Trick

 

[mastroego]

- Eye of the Beholder
- Tie Fighter
- Alpha Centauri
- Diablo 1
- Half-Life
- Planescape: Torment

Music, sound and environment mastery.
EoB might be a bit of a stretch admittedly, but its impact was enormous for me back then.

 

[SirSingAlot]

yeah, i was literally sweating in the spiders dungeons and was getting panic by all the sound they made in the dark corridorrs

 

[Sensuki]

Diablo 1 and Baldur's Gate 1 would be up there for me. Ghost Recon also had great atmosphere IMO.

 

[Zboj Lamignat]

No one mentioned Myth yet?

 

[Seerix]

From childhood, that would be Evil Islands and Sacrifice. I really really wish EI gets revived as an GOG title, this game doesn't deserve to rot in obscurity.

Nowadays... well, hard to come up with a game that doesn't have some kind of rpg elements, lol. Regardless, The Nameless Mod was freakin' awesome with its beautiful mix of abstract and surreal themes of virtual reality, internet forum and fuckton of references and jokes. Being probably larger than the game it was made with helped a bit as well.

 

[Great Deceiver]

Dreamweb. That game had an awesome soundtrack and the general mood was top notch.

 

[MRY]

As absurd as it is to say, no game's atmosphere has affected me the way the Doom Alien:TC did.