I love ambient soundtracks that invoke feelings related to the game, immmersing you into the action. The mystery of wandering through a towering forest, the adventure of exploring a new city, the unease of not knowing where the next enemy is, the calmness of staring into the sunset, the intensity of a firefight. Games that manage to do this very well tend to be some of my favorites, such as:
Portal
Mirror's Edge
FTL: Faster Than Light
Minecraft
Fallout 4
Skyrim
Half-Life 2