I'll agree on both Halo and Vampire The Masquerade: Bloodlines, Bloodlines has a great mix of original and licensed music. I've found myself listening to Shadow Warrior (2013) soundtrack on occasion, it's pretty solid on its own. The best though, to me, is Skullmonkeys' soundtrack. Like the game itself, the soundtrack is absolutely insane, over the top, wacky and simply perfect.

Dragon Age: Origins

And Diablo 1 isn't bad, specifically the Tristram music.

You can't go wrong with Stéphane Picq! He did the music of a couple of old Cryo classics like Dune, Lost Eden and Atlantis.

Tie Fighter!! :-P.
And I loved Discworld songs too. There is an enhanced version of this soundtrack here

Ah golden memorys especially the reminder of quake 1 thanks to Toxic Tom
and as there is also Morrowind already mentioned I have only Homeworld to add
for me the intro with adagio for strings and the exodus of a whole world is still one of the most appealing intros of gaming history.

three of my favorites are:

-Bastion
https://www.youtube.com/watch?v=oLA0vB9LCTM
-No more Heroes
https://www.youtube.com/watch?v=Mi469Fi7n3k
-Jet Set Radio
https://www.youtube.com/watch?v=4V4IEV8l-gA

Some lunatic put up a 10 hours edit of the Angry Birds theme on Youtube:
https://www.youtube.com/watch?v=GUQv4Pir0tQ

I wonder what would happen to people if they were forced to listen through this in its entirety.

Old but still badass. :^P

Oh yes. I got something to say.

I hope I don't come off as a bleeding heart here, but the song I'm about to present you is the story of my life. From the time I was born, all the way to the person I am today, no other song has illustrated or captured the very essence of my existence than this one.

Let your heart prepare itself to feel touched before you click the link.

https://www.youtube.com/watch?v=XVkjLyCnDSY

I f'ing love this

And this

Also throw any TLoZ ost up untill Wind Waker and I'll bite.

Honorable mentions : VVVVVV, Binding of Isaac & BoI Rebirth and Super Meat Boy.

I absolutely love orchestra adaptations of game soundtracks. Here a great clip:

https://www.youtube.com/watch?v=7IEqsHHbKkw

Monty on the Run is nuts!

Great clip! I need to spend more time on YouTube looking for other orchestras.

Can't send a decent YouTube link via mobile, however there are a ton if you search via "eminence Symphony Orchestra" - they add visuals from the games etc to some of the concerts however one of the great things is actually not knowing what's coming up then recognizing it. They also worked closely with yasunori mitsuda at one point, a Japanese composer for games.

Now imagine that in the German version of the game the marching sound and the voice sample had been removed for some reason. It took me ages to find out why everybody loves the Hell March or why it's even called that. The bass alone is an insanely crappy intro. I've loved it ever since I've heard the original version, though.

Reminds me, there was an epic concert in Cologne a few years ago. All of the compositions were by Chris Hülsbeck, the majority of them were themes from his old game soundtracks.

My favourites were these two:
Tunnel B1
Jim Power in Mutant Planet

Incidentally both of them are from games I didn't even know before I heard this concert.
Sadly I couldn't make it to the concert myself but I got the CD and have listened to it like a thousand times. Amazing stuff IMO.