Pretty much everything in Mass Effect 3 for me. The opening piece with the child and the Reaper is genius in particular however.
On a related note, check out this music video and song a very talented girl created from the Mass Effect 3 themes.. shame she used BroShep in the video, mind ;)
This one is still my favorite.
For me there's absolutely no question about it, it's "Vigil" Probably my favorit game soundtrack since Ocarina of Time and "Lost Woods" and "Zelda's Lullaby"
has to be the galaxy map theme