Heroes of Might and Magic 2

Kings Quest VIII: mask of Eternity

Doom.
The original game gave such a suspenseful background to the game, so different to the simple tones or music that had gone before. It both broke new ground and set the trend for future games. Yes, computers and programming have gotten WAY better since then, allowing programmers to do wonderful things, but few, if any, games have ever changed the boundries so much.

Out of many memorable game soundtracks I would love to boost, my favorite of all time has to be Curse of Monkey Island. The blend of swashbuckling pirate music and Caribbean rhythms is unique and so well executed. The wide range of tracks runs through so many emotions: rousing, relaxed, spooky, cute, irreverent, heroic, cool, and above all humorous. LucasArts games typically set a high standard with their music.
HOMM3 is also a longtime favorite, as are other Might & Magic titles.

Shivers

I picked this game up as a child from a market stall, not knowing anything about it, but finding the disc art compelling and figuring it would be something close to what I'd gotten used to from Lucasarts games.

It was nothing of the kind. It was terrifying, but so, so compelling. I used up entire stacks of post-it notes writing down hints and symbols I found scattered around the museum, and even though it scared me to no end, mostly because of the music that so perfectly set the correct scary mood for each area, I could not stop playing it.

It might not be considered a masterpiece of it's genre, but it will always have a top spot in my memory. I still have the original disc I bought back in the 90s, but it long ago stopped being playable on modern computers. I waited and hoped for many years that GOG would bring it back for me to play, so when it finally showed up for sale I jumped back in.

Even though I'm now an adult, and the graphics are very dated, hearing the soundtrack play immediately brought me back to those dark evenings playing this game as a child, and made me once again feel that terror no other game has ever made me feel. I was thrilled.

Banjo-Kazooie

My friend and I used to whistle it together.

For me Frostpunk OST. It's just so great. It fits peefectly into theme and makes you both sad, and desperate for survival, motivated and deluted of all energy, melancholic and patriotic.

Diablo II. As every time, when I hear it, I again get the chills of first facing Anduriel.

The Summoner (2001 on PC).

My parents always did their best, but still we were barely managing to make do with their paychecks. Sometimes it was tough.

I was 10. In November we were out shopping when I saw a game in a mall stand - it was called Summoner. I was convinced it was the best thing ever! Alas, I was told "maybe, someday". I knew that just meant "never", I figured my parents didn't consider it worth the money. Oh, the pain.

It made me cry when I found the game a month later under the Christmas tree. I was allowed to play it till 10 PM, which was really extraordinary. It was like the planets aligned that eveing and I was given a golden permit to have fun.

20 years later the music still evokes the feeling of thrill and loveliness in a munday life of rainy days.

Deus Ex (first one)

I still listen to it as it reminds me the events and locations in the game. The music was very well adjusted to what was happening in the game, one track even has the sound of wind in it ;) Years passed and now my children also love the soundtrack.

Castlevania: Symphony of the Night. The soundtrack was very well done.

World of Warcraft original was just so good. that or Diablo II soundtrack

Advent Rising

hard choice, but this one is probably most memorable as a whole, not just few tracks
it's a pity game was pretty bad

Mine is Darksiders II by Jesper Kyd (also like his work on Hitman series 1-4 and Assassin's Creed, till Revelations, didn't heard his work in Valhalla though).

I really like the difference in tone between Forge lands an Kingdom of the Dead specifically.

It is a shame, that my most favorite track, Guardian theme, was not released as part of the official soundtrack.

Other, which I really like are The Makers Theme, Into Eternity, Makers In the Outlands, The Corruption, The Makers Overworld, Crystal Spire, The Crowfather, Death Plains Theme, The Eternal Throne, City of the Dead, Plains of Death, Demon Realm and Avatar of Chaos (Absalom theme), which was also not released as part of official soundtrack.

Tough question impossible to have just one answer.

Atlantis II Highlands holds a special place for me. It works both as an introduction to the setting, a small island with a humble church, where old gods meet new one, but it's beautiful just listening it on it's own. I could just sit back and listen to it while the busy bees where circling the small island.