Akira Yamaoka is a fantastic composer, his tracks bring the really haunting parts of Silent Hill to a whole new level. Completely different sound, but Michael McCann's work on Dues Ex was superb as well. I find myself going back to both soundtracks often even though I don't actually play either game any more. There's just something about hearing the Icarus Theme while browsing on the internet that seems too hilariously climactic.
Hands down, though, Jeremy Soule is my favorite video game soundtrack composer. The guy's latest work for Guild Wars 2 and Skyrim deserve credit, both keep you really involved in their respective worlds and stay true to the in-game lore by using various instruments you can see in either game. His work on Oblivion and Morrowind, though... good god that is just magical. I can't even fathom how many times I've listened to Oblivion's soundtrack at this point.