I've always enjoyed the use of classical music in video games. Not soundtrack scores, but rather pieces which could stand on their own without visual pairing. Some examples:

Original Works
Echochrome 1 & 2: Classic string quartet pieces

Actraiser "Birth of the People": Baroque style piece with romantic/thematic elements

Masterworks Used In Games
Return Fire: Wagner's Valkyries (Helicopter), Flight of the Bumblebee (Jeep), Holst's Mars (tank), Grieg's In the Hall of the Mountain King (ASV), William Tell Overture (got flag)

Eternal Sonata: A game that is based on the imagination of Frédéric Chopin while he is on his deathbed, and uses his music as well.

In my opinion, it would be Chrono Trigger, Chrono Cross, and Titan Fall. I also like the soundtrack to Inquisitor when I'm in the mood for some slightly less engaging, relaxing background tunes.
Oh ya!!! I forgot about the Actraiser soundtrack. Good call!

Not to mention that Verdi's Requiem was used as the title theme! It was the first time I ever heard this piece. I was really blown away when Wargasm used it a few years later, though, the arcade mode where you would wreck havoc to it was pure madness.

some gameplay footage btw (albeit accompanied by Freude Schöner Götterfunke, also featured in the game). Also in Wargasm I stumbled upon [url=https://www.youtube.com/watch?v=EV92yt6sz34]Beethoven's Symphony 7, 2nd Movement for the first time. It played in the main menu and I think it may have been the first time that I actually truly fell in love with a classical piece of music. The game was just loaded with amazing classical music.

Metal Gear Rising: Revengeance had an amazing soundtrack. :D

Then you would probably like Apidya too. For me those two are the best of the best of what Chris ever did. I really like how that soundtrack works: Each world has three levels and the worlds have a basic theme that is varied and remixed through the levels and bossfights. Very cool.

Outlaws seconded! Also a very recommendable, the soundtrack of Shattered Steel, also by Mark Morgan IIRC.

And oh yes, MDK by Tommi Tallarico also comes to mind.

I agree, DMC 3 soundtrack is very good too.

I never noticed but it's been like twenty years since I played it, I didn't pay attention to such details when I had just begun going to school. :p May check it out sooner or later, though. Sounds interesting.

And actually not that related but reminds me, Speed Busters has an interesting music concept. Each track has its own music and each song is further divided into loops which correspond to specific segments of the track, so different music plays depending on where you are. The music itself isn't groundbreaking (but definitely cool) but this system honestly blew me away. It's amazing how much more intense this whole wacky experience gets due to this. I haven't seen this in any other racing game yet.

I liked the music a lot but it doesn't come close to the MDK2 soundtrack, IMO, at least the Jesper Kyd parts.

Oh yes, how could I have forgotten Verdi's Requiem in the title! That totally set the tone for the whole game.

I hadn't heard of Wargasm before, thank you for mentioning it. I guess classical music and war do go pretty well together. I think Apocalpyse Now probably had a hand in creating that oft used pairing.

I also have a puppy's crush on Yoko Shimomura.
I wish I was her neighbor. I could listen to her music all day.

Legend of Mana remains to be one of the best soundtracks for my ears.

Some random games, skipping ones already mentioned;

Guild Wars 1/2: Soule has good music, too bad he's insane. They have started branching out now that he no longer works with them.
Beyond Good and Evil: Some really nice tracks.
Minecraft: No really, it's actually rather good music.
FTL: On their own they are really good, in game when it transforms between normal and battle music it's just wonderful.
Smash Bros: Many of the songs be them remakes or new have been rather good over the years.
X series (egosoft): hehehehehe, ear candy.

By a mile my favorite is the soundtrack from the newest Devil May Cry. It is done by Combichrist which is my favorite band lol so naturally it rules to me!

Agreed, I don't like that kind of music in any other context, but I loved in while playing DMC.

Combichrist rules

I favor many soundtracks and some of them are: Diablo 2, Divine Divinity, Divinity: Original Sin (I really love it, sometimes while playing, i stand in place doing nothing just to hear the marvelous tunes), Skyrim, Baldur's Gate, Icewind Dale & of course Anno 1404 (meditation mode: on).