Heroes of Might and Magic II
Many weeks of playing with it's soundtrack.

Gotta be Diablo/Diablo II. Matt Uelman's eclectic mix of wind instruments, guitars, percussion, and other sounds is wholly iconic and instantly brings me back.

Grim Fandago!

The original Homeworld. The third mission and the perfect use of Adagio for Strings took me by surprise. Until that moment I really didn't think much of video game music and well they should fit with the game.
I've had some newfound love to video game OSTs over the years and gotta say that there have been absolute bangers over the years but Homeworld OST will forever be the one true OST for me.

Command and Conquer Red Alert 2.

I still fire it up occasionally 20 years later because it's just good rock music. It reminds me of frantically trying to build a base and fight off the opposing side with a variety of awesome units.

The first Red Alert soundtrack will probably stay in my head until I die.

Fatal Frame/Project Zero saga

Simply one of the best soundtracks you can find in horror games, especially 「蝶」from the first version of Fatal Frame II, is just an amazing track

Gonna have to go with Chrono Cross's "Scars of Time". I never really got into Chrono Trigger, and Chrono Cross was interesting, but that opening cinematic theme made me burst into tears the first time I heard it.

The wistful opening is so obviously filled with an unstated regret and longing. It's a palpable sense of loss, one that cradles you and sets you up for the punch of the rapid-fire, overpowering syncopation of the second part. And what a second part it is!

Catchy, yes, and full of determination. I always feel like the best music tells a story without an explicit narrative, and Scars of Time is very clearly a modern opus of exploration and discovery in-progress. The thing that gets me is that, musically speaking, it doesn't really conform to folk music or 'island' music, though it carries some of the theming and instrumental ideas found in these genres in our own world.

It's [I]Exotic[/I]. It's [I]Alien[/I]. And for all that, it's also incredibly approachable and relatable. It goes DEEP.

Chrono Cross. Before youtube I would often turn the game on just to listen to the intro music or pause the game at a spot where one of my favorite bgm was playing and let it repeat for a good while. The music is also used very effectively in the game. The opening invokes excitement at the journey you're about to begin. Both versions of Arni village invoke a quiet and relaxing village atmosphere, while the world areas invoke a feeling that suits the area, from nice jungle beats for your jungle adventures, jovial port town music, and more.

I have many, many game soundtracks, but the most memorable for me is Jon Everist's score for Harebrained Scheme's Battletech game. It's stunning, powerful, and adds a cinematic flair to the game that fits perfectly.

Diablo 2

When I was a kid a had a demo of this game which only included one class and only two first missions. I would play it over and over again until one Christmas when 20 years younger me opened that box and unveiled the Diablo 2 + Lord of Destruction. From time to time I play the soundtrack once again to feel that bliss nostalgia and unnerving feeling of unknown that lies ahead.

Heroes 2-Opera glorious opera. Beautiful for its time.

I think the most memorable video game soundtrack is E1M1 from Doom.

Grim Fandango

E1M1 from Doom because it gets your blood pumping.