I don't really think there is any one "best gaming music", because such a thing is incredibly subjective. For example, many have mentioned Still Alive from Portal, yet I absolutely loathe that song. Meanwhile I praise the other piece of the Portal soundtrack which go largely unnoticed by others. So I guess this thread is essentially a "favorite game music" thread, and I will respond appropriately. Though personally I don't even have a single piece of favorite gaming music since there are so many different ones that mean different things to me.
However this is one of my personal favorites. It's a very immersive piece from both of the Half-Life games. During the first game it played during the tram ride into the heart of the facility. The time at which nothing was going wrong and you may have forgotten that the game was supposed to be about a physicist bludgeoning aliens to death with a crowbar. It really eased you into the game and helped you to assume the role of Gordon Freeman. In the second game, it plays when you acquire the gravity gun in the scrap yard with Alex. And that's what I find to be special about this song. During both games, it plays in the only two area that could truly be considered "safe". It invokes a feeling of calmness that is experienced exclusively in these two areas, a time in which you weren't being told to hurry along to the next destination. The music is used to create a very special moment of nostalgia for the player as they think back to the very beginning of it all.
The area this song plays in is very different from the above. In Half-Life, this song plays inside a teleporation room in Xen, while in Half-Life 2 it plays in the flooded basement of Nova Prospekt. Despite both of these areas being in the heart of enemy territory, this song also invokes a feeling of calmness. You know that you are still in danger, yet these places feel relatively safe. A calm during the middle of the storm.
These two songs invoke feelings that contrast with the overall oppressive feeling of the rest of the game, hence why they are memorable to me. This at least how I felt when listening to them. I imagine people had different responses to these songs than I did.
