WoW, hands down, bar none, absolutely no competition whatsoever.
Blizzard is a true master at creating atmosphere, and no where did I notice that as much as I did in WoW. From the environments (low graphical quality doesn't mean a game can't look stunning) with their minute details (little veins covering the flowers in Duskwood, Grizzly Bears catching fish in the river in Grizzly Hills) to the soundtrack, it all worked together flawlessly to create a fantastically immersive world to play in.
And WotLK was the absolute crown on their work so far, I thought it couldn't get any better when I walked through Terokkar Forest. And then I left the boat and entered Northrend. By the heavens, I was flabbergasted. It already started with the breath taking Howling Fjord:
You leave the boat and WAM, it sets the mood immediately. So epic.
I wowed when I flew into Scholozar Basin:
Soft and exotic tones, as if they're from another era, perfectly suited for the excluded jungle in the Basin.
But the absolute number one must be Grizzly Hills:
It still sends shivers down my spine each time I hear it, even though I stopped playing over a year ago. Each time I neared Grizzly Hills, I would step off my flying mount, and just walk through the forest while listening to those stunning tones.
As you can see, I'm quite the WoW fan, even though I don't play anymore. Gods I love you Blizzard.