There are literally too many soundtracks from games that are my favourites, such as Gears of War, Halo, Fallout 1 - 3, but if I was forced to choose, I guess Mass Effect 2. Jack Wall did such an amazing job in capturing the epic scale of what was happening and to actually make us feel and care about it through the music, and he was able to expand on what was an already excellent soundtrack for Mass Effect.
Of course I realise all composers and soundtracks are supposed to make you feel and care about what you see, it adds depth and emotion and all that, but I think the soundtrack to Mass Effect 2 did it better than any other.