Daikatana: The first few levels of episode 2 (Dark Age Norway).
Unreal: too many maps to list. 

is the most atmospheric game ever
almost every map there has it's own mood and feel.

i liked some of half-life maps, which have nice abandoned base atmosphere.

atmospheric games: unreal1, ut1, rtcw and more 

Enclave:
So many maps, especially the port and the ancientish map with that big zeppelin. I love Enclave very much.

Quake:
lots of them!
czg's maps (czg05, czg07, czg04, neh2m2 and the winner is neh1m5), vondur's maps (koohoo, neh1m9), qurnel's maps (stormland, north, mine), tronyn's soul of evilish maps, frib's castle maps, than's very unrealish a2 and a3 maps etc.
also, we should mention casspq1

Quake 3:
bal's golconda (i don't remember it's bal3dm1 or bal3dm2), lunaran's lun3dm1 (very desertish, yummy), etc.

Half-Life:
Some maps at start, when the base is damaged and destroyed. Pretty scary atmosphere and nice details, too bad later they've lost it.

Redneck Rampage:
most of maps =) especially the hospital for retards

Operation Na Pali:
lots of maps, i don't remember names, but afair the 8th and 9th map owned me slightly (caves with these bright crystals)

Unreal Tournament:
morpheus, galleon, some castles of names I don't remember and so on =)

Unreal Tournamnet 2K3/4:
Blah, dozens of great designs!

Hexen 1:
the first and second map, the swamp (8 map iirc)

okay, too many of stuff to write down =) 

redneck's hospital scared me so much.... 

I had the first reply to this thread, but it has vanished overnight, bizarre. Either way, I wanted to add the first few levels of DKT episode 2 (Dark Age Norway) to the list as well as Unreal (too many maps to list). 

it's still there, adjust your lens properly... 

rich_bar, for me, really captured the true essence of a night out drinking with friends. 

wins for me, with Quake 1 running a close second, followed by Half-Life.

Even Unreal's botmatch levels had more atmosphere that most SP games. Unreal's simply awesome sound effects had a lot to do with that. 

It'd take awhile for me to recall all the atmospheric maps, but Blood definitely had several despite its old tech.

I recall a Quake 1 SP map, I think it had "Cassius" or "Cassy" in the title - you were on a cargo ship that had stowaway aliens or monsters onboard. I never did complete that map though, it was so confusing. But the whole time I played it I never found one monster, so that added to the suspense of not knowing when you might encounter something. Couple that with the dark halls.. 

Hell in a Can is what you're referring to. 

...and I pretty much agree with a lot of stuff that's been mentioned, Unreal in particular.

Will have a think about other stuff but so far:

Blade, FAKK2, Enclave, Undying & Deus Ex spring to mind.

Actually out of those, Undying is probably topping it....the time-changing monastry scenes and eternal autumn and that other dimension in the blood-red void in particular.

Quake maps - the usual shizznazz you know. 

Darkmere from Hexen, it's what pushed me over the edge to want to map. Or is it Shadowmere? I don't remember. 

in a Quake oriented forum and noone has mentioned Zerst�rer - Testament of the Destroyer yet? Forshame -- the start map and the end map both are chilling and reek of splendid evile. 

Unreal was atmospheric, at least the first levels. I reaaaaaally like Dm-Ariza, seems much more atmospheric than any Unreal map (yes in any game) since. What a brilliant piece of work.

And obviously, the first two maps of Unreal SP are moody, and have a sense of terror and wonder respectively. 

I'm currently playing through Painkiller, and many of the maps in there just ooze atmosphere. The Snowy Bridge map was pretty sweet, and the abandoned Military Base was very nicely done. The ambient sound in Painkiller kicks much ass.

I very much liked the Rebellion map in Far Cry--when you come out of a long underground stretch into the open again, dawn is just breaking in the distance, and you're surrounded by distant echoes of gunfire and men and trigens screaming and yelling. Masterful level.

For Q3, anyone who hasn't seen it should check out Glassman's latest level, Dead Souls (gm3tourney2). It perhaps has more atmosphere than any Q3 map I've seen. 

That iam1.bsp (if anyone knows what im talking about) blew my mind the first time i played it. Like the giant cross and whatnot? Its the first time i recall myself just sitting back and saying... "whoa" over a quake map.

I also like "zuggernaut vertigo" and
"The wind tunnels" Yeah unreal1 was cool too. And alice. To be honest im sorta a FPS ahmish type dude i havint played most these game you mention. 

any thief1/2 level. 

I'm unsure whether "being scared into God mode" is a symptom of atmosphere or not. 

soe2m5, ap1sp1, sgodrune, sadlark5, oum4, toum, czg7c, dazsp2, ls 

Zerstorer[hope i spell it right]is THE masterpiece and shall not be forgotten :D
I think the second playable map(...in the dead)is quite atmospheric, hmm... 

...we consider ourselves suitably reprimanded for that shocking omission! 

iam1.bsp(Q1) yeah that really blowed my mind, that your "own jard" changed to void.. unbelievable! I tried(years ago) to send thanks and cudos to mapper, but hes email didnt work. I hope he is reading this. I myself am editing my first Q1_bsp, I have some cool ideas, I�ll do my best I can relelase it as I think it in my head.
>That iam1.bsp (if anyone knows what im >talking about) blew my mind the first time i >played it.