Europa Universalis II
There is nothing that puts you in a more combative mood than classical music.

The Mass Effect soundtracks.
There are many games where the soundtrack has been well done and supports the on screen gameplay or story etc. But the ME OSTs are the ones that stayed with me over the years.
Toasted a CD, listened to it in the car while driving on the Autobahn.
Played it while doing something else.
Actively listened to it with earphones while lying down.
Not all tracks are great, of course, but the majority of them are.
Even someone who doesn't play games but knows sci-fi movies immediately knows or feels the setting.
Just writing this makes me want to replay the games now ;)

As my most memorable soundtrack (and that I still actively listen to) is Heroes of Might and Magic 4. While gameplay might have been different from it's predecessor, it's music was even more enchanting. I played it on the year of it's release and I remember that few times I left the game on while doing something else next to my PC, just to listen to music. In fact, I did that with HoMM 2 & 3 aswell, but #4 was still the most impactful.

After original HoMM-series (NewWorldComputing), I've been a keen fan of Paul Anthony Romeros work, though I haven't played any of them after HoMM 5, I've still enjoyed their soundtracks. BUT, the original HoMM-series music has that emotional bond of my youth and listening to this amazing music while playing the game.

Another very memorable was when I was very young, my brothers and I were playing Last Ninja 2 on Commodore and the Central Park -theme left an impression on my mind. That memory returned few years ago, when Matt Gray began to remaster old Commodore and Amiga soundtracks to todays standards. Excellent work from his part.

My favourite one is Horizon Zero Dawn's music. Its so beautiful, even its mirroring the game itself with promising some adventure, incredible discoveries. One of the intersting part of this, the music canmake you feel this feelings without any vocal singing, just the pure music itself. The whole album is a standalone game with a lots of feelings like happiness, sadness, loss and fighting. Its just pure magic.

Donkey Kong Country 2

I've listened to it over and over in may different variations.

The one symphony version that was done in full is fantastic.

It used to be one of my greatest assets in studying for exams in college.

A Heroes II Vinyl would be amazing too.

Wing Commander Prophecy is my soundtrack of choice. It's a nice collection of fast paced music of different genres, fitting well with the fast paced space fighting from the game. From Rammstein to Junky XL, from Cobalt 60 to Juno Reactor. It's great mix.

Mass Effect Trilogy. Perfect mix of intense combat music and chilling/beautiful music. Perfectly displays the hope and hopelessness of the Reaper invasion

In my opinion Heroes 3 of Might & Magic 3 has one of the best or the best soundtrack from all games. I can't say which theme is the best because they all are! Each of them perfectly matches to the town theme.
I once did a competition to my fiance to match the soundtrack to the towns (without knowing the game) and she almost matched all of them correctly - this says how good were they composed and created.

I often listen to the whole soundtrack during work and I love it!

I'd have to say most memorable soundtrack has to be from Metroid Fusion. That game held so much tension and dread in its run time and sooo much of it came down to the choices of music used, especially Silence 2, after the power is knocked out. You spend the entire time positive your about to run into SA-X

It's a tough choice between Max Payne and GRIS, but I'd go with GRIS. While the main theme from all of Max Payne is an iconic OST and is an memorable part of my childhood, the GRIS OST is just out of the world. It's soothing, adds a sense of rush, add every emotion to the game, it's just one of the best I've listened to.

I think I'd have to go with Might and Magic 6. I keep going back to that one, and the New Sorpigal track gets stuck in my head any time I hear it. However, this was a hard toss up between Mega Man 3's OST.

Surprised no one has mentioned it yet, but for me the game with the most memorable soundtrack was probably Silent Hill 2. I loved the soundtracks to all of the first four games (I prefer to not think about the rest), but the second one made the biggest impression.

It wasn't just the alt. rock parts (although as a teenager at the time I was primed to be into that kind of stuff), the "scary" part of the soundtrack also blew my mind with how effective it was at creating an unmistakable atmosphere. How it didn't just convey dread, it also caused it!

The only other music like that I was aware of at the time was Aubrey Hodges' soundtrack to the PlayStation port of Doom, and that one creeped me out enough as a 10-12 year old that I wasn't able to play the game without having my father in the room with me. :D

I'd say Red Alert 2 (+ expansion) - hard hitting, high octane, really memorable. It also meshes well with the unit voice lines.

That would be heroes 3. Me and my father have gamed together for as long as memory goes. And while we have tried new things as time gone buy, each year we always fibd time to play a few matches of the game. When I was small, he played it like others do solitare and the music was audiable through the appartment. It has been with me all my life and on my playlist on my phone. Favorit track is necropolis themesong. (best theme and team.)

For me is The Secret of Monkey Island, I love the music.

It brings me memories of playing it with my aunt and my brother, trying to beat it.