I had a quick flick through and don't think anyone mentioned Deus Ex HR. Aside from Icarus, the entire soundtrack of the game is outstanding- on their own some are really weak but, for me, the real triumph of the DXHR soundtrack is atmosphere. It really helps build on the atmosphere of the argument. Icarus is an outstanding stand-alone piece (imo) and the opening credits track (Forget its name, sorry!) really struck me as well.
The MGS series has been mentioned several times here and I'd like to echo that, having replayed the HD collection recently. Not only is the music memorable, but there are "center pieces" in the game(s) as well. For example, I played Peacewalker recently and on one of the latter missions the song "Heaven's Divide" played which really cements the game in my memory.
Someone mentioned Skyrim too which I couldn't agree with more, the soundtrack there really made the game stand out to me.
I think that it's important we don't get too hung up on "memorable" tunes, though. They're great, they have their place, but what about music that simply adds to the atmosphere? Gaming is an interactive medium so music should (ideally) add to the atmosphere and be able to impart a.. sense of the game world, of the surroundings. You've got your visuals, you've got the nearby sounds (NPC's coughing or what have you) but what is the baseline? What is in the "silence"?
Games that pull of both, having a hugely memorable soundtrack that completes the game's aesthetics and delivers part of the story with it is truly the best, but being great at one or the other isn't too bad either.
I can think of examples of truly "bad" music such as the recent Sonic 4 Episodes 1 & 2 which I thought sounded terrible, but I don't think that'd leave me to say game music overall is "bad". If anything, I'd say it's way beyond what we had ten, twenty years ago and has made some of the really stand out pieces in modern gaming stand equal to, if not better than the old 16 bit tunes.