It's great to see game soundtracks getting some love, they're overlooked far too often to my taste. As an avid game soundtrack listener (and I have been for some 15 years), I've quite the collection, but I tend to organize them by composer, rather than by game. And since Ms. Spencer asks, here are my favorite, with a few links to the tracks which I feel best characterizes the composer's music (I chose to link rather than embed, because this is turning into a lengthy post).
Mark Morgan - My all-time favorite game music composer, he's worked on Zork: Nemesis, Zork Grand Inquisitor, Planescape: Torment, as well as Fallout 1 & 2. The haunting themes he created for Zork: Nemesis [http://www.youtube.com/watch?v=tWLr8Qb3jzU] are just permeated with the dark, oppressive ambiance of the game and contributed, beyond the shadow of a doubt, to the atmosphere of the game.
Jeremy Soule - A very close second, his works include Icewind Dale, The Elder Scrolls 3: Morrowind, The Elder Scrolls 4: Oblivion, Guild Wars, Neverwinter Nights, and "Star Wars - Knights of the Old Republic". There's no denying he's one of the cornerstones of the game music industry, and his reputation is well-deserved. I'd be hard-pressed to pick my favorite out of all the albums he composed, but his work for the Elder Scrolls games [http://www.youtube.com/watch?v=rtvVQn6Wbyw] is probably the most well-known.
Michael Land - He has mostly worked with LucasArts and composed the soundtracks to the Monkey Island games (my favorite being The Brimstone Beach Club [http://www.youtube.com/watch?v=XjbGlYBssk8], it truly gives off this "tropical island" vibe), but also the Indiana Jones and the Fate of Atlantis, The Dig and Star Wars: X-Wing.
Jack Wall first came to my attention in 2001 with his stunning soundtrack to Myst 3: Exile, and since then, he's done nothing but deliver. With the scores to Myst 4: Revelation, Jade Empire, Mass Effect (both 1 & 2), he not only proved his brilliance, but also his incredible versatility. It's difficult to select a single track which best portrays his style, as he has composed so many different scores, so instead, I'll just point you to first track which ensnare me, the opening track to the Myst 3: Exile soundtrack [http://www.youtube.com/watch?v=XFw0gbQk23M].
Peter McConnell is another artist who worked a lot with LucasArts, and he even worked hand in hand with Michael Land on a few games. The soundtrack that stands out, however, is Grim Fandango, as shiajun [http://www.escapistmagazine.com/forums/read/6.194838-Catherine-s-Soundtrack-Renaissance#6252023] already pointed out. The entire soundtrack is a masterpiece, ranging from mayan flutes to jazzy saxophones, but my favorite is High-Tone Fandango [http://www.youtube.com/watch?v=4VZUxci9u54]. Incidentally, this is one of the rare game soundtracks that my non-gamer friends enjoy.
Inon Zur has been getting a lot of publicity recently for Dragon Age: Origins, but he's been active a lot longer than that, and for while, I was beginning to think that he would be forever known as a composer of what I dubbed "sequel soundtracks" (in the sense, that he would compose soundtracks to games that were the sequels of a game which had a powerful soundtrack composed by someone else; am I making any sense?). Icewind Dale 2, Syberia 2, Baldur's Gate 2: Throne of Bhaal, "allout 3 and EverQuest 2 are the ones that stand out, but all of his work has been near flawless, and I'm glad he's sharing some of that limelight.
Michael Hoenig - Speaking of Inon Zur's work has led me to chilling realization: I forgot Michael Hoenig who gave us the soundtracks to Baldur's Gate and Baldur's Gate 2: Shadows of Amn, which were absolutely epic, suiting perfectly games that were also epic, and set the benchmark for all RPGs to come. Here's the main theme for Baldur's Gate 2: Shadows of Amn [http://www.youtube.com/watch?v=U88zyFLzIXQ]. 'Nuff said.
I've still a few on my list: Bjørn Arve Lagim (The Longest Journey), Tor Linløkken (The Longest Journey), Paul Romero & Rob King (Heroes of Might & Magic), Mark Griskey (Star Wars: Knights of the Old Republic 2: The Sith Lords), Jesper Kyd (Assassin's Creed 1 & 2, Hitman), Chris Brayman & Mark Seibert (who worked on many, many Sierra games; see Andy_Panthro's post [http://www.escapistmagazine.com/forums/read/6.194838-Catherine-s-Soundtrack-Renaissance#6244237]), Tim Ebling (a few Ecco the Dolphin soundtracks come to mind, perfect for working or relaxing), Leon Willet & Simon Poole (for their work on Dreamfall)... The list goes on and on, but I figure I've taxed the reader's patience as is. Anyone who's read this far gets a cookie though!!