AGH WHAT WAS THAT- Oh, it was just the wind. AGH WHAT WAS THAT-Oh, it was just a rock falling. AGH WHAT WAS THAT- Oh, it was just weird mumbling sounds. AGH WHAT WAS THAT- Oh, it was just the wind. Etc. x]

STALKER deserves an honorable mention. The 'soundtrack' is pretty low-ley, which helps to set off all the interesting environmental noises. Footsteps, fireside guitar, moans, growls, clanks, radio...

Thief had amazing ambient and directional sound, especially for its time.

Morrowind/Oblivion do as well (although theirs can be aided by fan efforts). Approaching the Dark Brotherhood door in Oblivion, approaching the arena and hearing the cheers increase in volume... good stuff. (and they had some ex thiefers on board to help ;) ).

My pick is Cinemaware's 'It came from the desert' back in 1989. Amazing music then, mind blowing now. The stereo effect was awesome, but transform that same sound to 5.1 or higher, Wow.

So much fear with the souns of the zone.

Machinarium.

The music was fantastic, and the sound effects fit just about perfectly.

Funnily enough, I'm discovering age of wonders, and it's the first time i ever was struck by horribly bad music and sound effects in a game. Generally, when they're 'meh' i just don't notice them. In this case, I managed to be shocked.

I play it with the sounds off, now.

Okay, not exactly the expected kind of exemple (i'd say penumbra/stalkers, as already mentionned by others), but, well, just a coincidence to see this thread now.

I think that the ambiance from Battlefield: Bad Company 2 is really impressive, and I think it really adds a lot to the experience. Amnesia also does a great job of sucking the player into its world through the use of sound. Play it with headphones. There are also a bunch of Genesis games with kickass chiptune music that I really enjoy.