When I was about 9 or 10 years old, my friends were pretty fond of sharing some game soundtrack. I was very confused and wondering why some people like listenting something other than fine-made music. However, when I entered the main base of Haven in HMM5, I was shocked and lost in the hallucination the background music, together with the 3d scenery, had made. After that, I cannot stop aquiring that music by begging my best friends to download it from websites since I had no way to access the Internet.
That soundtrack stays in my mp3, mp4 and cellphone for raughly 9 years. Every time when I play some garbage games or unrecyclable cliche, that rhythm will begin to loop around my head. It remind me of the finest time of playing video games, even though I never know the name of that soundtrack.

https://open.spotify.com/track/5pl9SgFOuFoEVsrUYKORvC?si=7f26d4a04b4345f0

Star Wars: Knights of the Old Republic II: The Sith Lords

None of the Kotor music is repurposed Star Wars music iirc, and the music of Kotor 2 especially really conveys the sort of ominous and demystified feeling of being in its world. The whole gimmick of the game is that you're the last Jedi, and the soundtrack really hammers home how lonely and hunted you are. Special mention to the Peragus music because I've heard it so many times and I love the sort of nightmare scenario the player is thrust into immediately.

System Shock 2

That eerie beat when you're walking around the corridors of the deserted space ship, and intense techno that rises up when a mutant spots you and starts chasing you has certainly gotten my heart to beat the same bpm on more than one occasion.

As for the co-op game, System Shock 2 fits very well too. Nothing nicer than to hear your friend scream after entering a room and hearing a helpful servobot explode blowimg rhem to pieces. While you're laughimg at them you hear whirr of servos behind you...

Icewind Dale series

There are many game soundtracks that i loved over the years.

If i had to pick one that still give the chills would be the Icewind Dale series. I still listen to it especially when i roll a character for my DnD sessions!

Oh my, this is one touch question to answer.

I've been playing for quite some time and I pride myself on "getting into" the games and music is a very big part of that.
When I started gaming in late 1998 I had access to quite limited list of games, however each and every one of the games I played early had amazing music that added depth and feeling.

In addition to HoMM3 itself I really loved C&C Tiberian Sun music, also Commandos: Behind Enemy Lines had memorable soundtrack. But as to the overall favorite, I would have to say that Baldur's Gate 2: Shadows of Amn will steal the show for me, the music was just that epic, and is to this day.

Runnerups would include: The Withcer (1)
Might & Magic 7: For blood and honour
Also recently I have a weird craving for the original Transport Tycoon music...

Max Payne. The only game I've ever installed again just to run it and listen to the main theme (until eventually downloading it from somewhere, anyway). Still my personal theme.

2 come to mind - Age of Empires and The Witcher:

Age of Empires probably because it is my childhood game and because the main theme is great even today:

The Wticher because it is the first game I was so hyped for and it really made the world come alive

Witcher, I like how some parts give hope and others just say "cold, hunger and inquisition" :)

DIablo 2 soundtrack is the most memorable for me.

Honorable mentions are:
The Witcher series
Max Payne 1&2
Heroes 3
World of Warcraft

Baldur's Gate. Maybe because I played this game more times than any other, but I have been in love with its OST for over 20 years now. I always keep it on my phone, and sometimes even use various tracks as ringtones. Main theme, combat and tavern music are the higlights, but the slower and more melancholic exploration themes always make me want to fire up the game and travel through the Sword Coast once again.

Wrath of the Righteous became an instant classic with its Mythic themes. They are real works of art.

Other mentions: Morrowind, Diablo 2, Starcraft, Dawn of War

Battlefield 1942. Man that theme song set the mood well.

hard to choose only one

Woud say Age of wonders the first Game
the ambient music calms me down
and i can listen to it for hours not getting bored of it

I couldn't recall the fortress theme off the top of my head so I looked it up on Youtube.

This guitar arrangement of that theme is amazing:
https://www.youtube.com/watch?v=BuwWUQRuR70