The atmosphere in a game can be just as memorable as the story or be a character itself. Fallout 3 proved this as the Capital Wasteland was a character all on its own. A great atmosphere is something that can draw you in and make you feel a part of the world.
God of War
The atmosphere in a game is supposed to suck you into its world and make you feel exactly how the characters are feeling. God of War did this best with its cold and dark environments and massive scale levels that are out of this world. The beauty of this Norse realm was incredible to take in and each realm really made me feel like I was actually there.
Runner-Ups
Red Dead Redemption II
Kingdom Come: Deliverance
Battlefield V
Call of Cthulhu
thebinarymessiah.com 
Yep! The fact that I forgot about this game until you made a comment proves that.