Glad to see the Mass Effect series getting some love. Honestly there were so many great tracks that perfectly captured emotional moments in the series that it would be hard to just pick one.
A somewhat underrated one as far as the ME soundtracks go though has to be the track that plays on Mars...
Shepard & the crew of the Normandy forced to flee Earth when the moment they've all been dreading finally comes, and the Reapers invade. The fleets sent to protect Earth utterly annihilated, whole cities being reduced to rubble, millions dead, and extinction is looming. Shepard and the crew barely have a moment to even process the scale of the disaster when they're sent to Mars in a desperate race against time to find salvation, only to find on arrival that indoctrinated minions of the Reapers have possibly beaten them to the punch.
That track perfectly captured the tension and sense of dread set up by the story.