The first game I paid attention to the soundtrack of was Nier back in the PS3/360 era.
The orchestral score was fantastic, with sombre themes conveying the state of the world you were in and grandiose scores when fighting bosses or other thrilling scenarios. The soundtrack featured lyrics combining elements from several languages, a nice touch given the game's setting.
It's sequal, Nier Automata, also had a fantastic soundtrack, but Nier's stands out as being the one that first got me to appreciate the sountrack in games.

The most memorable, I honestly don't know. I can only say that without its soundtracks (and the narrator), Bastion wouldn't be a memorable game.

The Quake soundtrack by NiN - I have soo many fond memories of that music pumping me and my friends up in Quake LAN parties at college. Such great times!

Moo

It’s so hard to pick one. But I’ll go with Final Fantasy X. The Zanarkand track invokes overwhelming feelings every time I hear it. In fact, just thinking about it now makes me simultaneously happy, sad, nostalgic and feeling a bit lost just as the game had Tidus feeling. This is my story… and I’m sticking to it.

Most memorable is kinda tough, often there are specific songs that pretty much define some game moments, even though the rest of the soundtrack might be more generic... but:

Concerning the overall game soundtrack... id say its a tie between Planescape Torment and Riven: The Sequel to Myst for me. PST has a pretty unique soundtrack for a fantasy game anyway, but it fits the tone and quirky setting really well i think. What i like the most about it is the reappearance of the main theme in several variations, some more epic, others slower and darker, worked really well i think and enhanced the atmosphere immensely.

The Riven OST is more subtle, with many quite ambient/droning sounding elements, it created a pretty foreboding, ambiguous atmosphere, even if the visuals where sunny and mostly peaceful. Pretty memorable i thought.

Just for fun, some runner ups for single songs/tracks:

Tristram theme from Diablo. Someone has it stuck in his/her head after reading this sentence, im sure ;)
Unreal Title/Castle Flyby track. Loved it back then and still do, great intro to a great game.
Deus Ex - Unatco HQ Theme... and Hong Kong....
Myst - Unfinale. Has been my phone ringtone for the last 9 yrs. I guess i like it ;)
Irene Goodnight (Bioshock Infinite)... especially when taking time in the beginning. Burns in.
Little Nightmares Theme/ Six's music box. That one stuck for days...
Witcher III Hearts of Stone Theme/Variations. Quite melancholic and really fitting to the story.
Monkey Island Main Theme and Stan's Previously Used Coffins.
Ori and the blind forest Menu theme. Whimsy. Awesome.
Baldur's Gate II: Throne of Bhaal - Fire Giant March. Could be straight out of Conan or other 80s fantasy movies, love it.

There are so many good answers to this question. Two games immediately came to mind: Undertale and Hollow Knight.

I know it can be hard to take this game seriously after all the memefication and bizarre fanbase, but the music here is second to none. Nearly every boss has a song more memorable than the best tracks of most games. Toby Fox's reuse and remixing of themes thorughout and his ability to evoke emotion through music is simply brilliant.

Hollow Knight's tracks don't quite jump out immediately in the way Undertale's do, but the way they enhance everything from the sense of wonder and exploration, to a feeling of doom and dread when a new boss appears is unmatched. I still get goosebumps walking into the City of Tears for the first time on a fresh playthrough.

For me, it is the soundtrack for Pathfinder: Kingmaker. I've always been impressed by it's ambiance, which totally relaxes me. :)

The Max Payne main menu theme just hits right in the feels not only that it was an amazing game with a good story but also from the point of view that it brings back a lot of memories from back in the days when internet cafe's where a thing, full of lan parties. The late night gatherings full of people who share the same hobby. Every time a new video game would launch there'd be a bunch of people gathering having a good time, amazing times, heck lan parties are amazing in general ... we still do them every now and then, few times a year.

Ah, such a tough question. Do you judge your favorite by how it impacted your favorite game or by how it made you feel as a standalone outside a game? Or something else?

From a purely technical perspective the iMuse system from Lucasarts was amazing and really brought something to their games.

The original DOOM was such a perfect kickass accompaniment to the gibbing and got your blood pumping.

The Heroes of Might and Magic II and III soundtracks both are masterpieces, not only elevating the games themselves but also most importantly worthy of simply enjoying while laying on the couch staring up at the ceiling. But, since this is a contest about those games I'm not going to go the easy route.

I'm going to go with the original Homeworld. I fondly remember sitting in my dark PC game room, good ol' beige desktop and a massive CRT monitor bathing me in its glow, Creative speakers cranked up as I fired up Homeworld. That opening music after the story cutscene.

Chills.

The chorus, the radio chatter, the epicness of the Mothership launching....

More goosebumps.

This was Cosmos mixed with Star Trek, this was a mind shattering experience, this was what I wanted from the first time I played Space Invaders. A game that put me right there in the ocean of the void, commanding starships while the Heavens praise our victories.

The Homeworld soundtrack works as a game backdrop, as a perfect sci-fi theme, and an emotional catharsis.

My most memorable video game soundtrack has to be Morrowind's.

It was my first open world rpg where I spent many days exploring the landscape. The enchanting melodies were just delightful.

I would say its either Morrowind or Baldur's Gate for me just some iconic music that I still remember when I hear it.

All-time memorable soundtrack is Duke Nukem 3D, because it was one of my first games played.

Other favorites that struck a chord over the years: StarCaft 1, WarCraft 2-3, Carmageddon 1, Mortal Kombat: Deadly Alliance, Max Payne 1-3, Diablo 1-2, Tony Hawk's Pro Skater 2-3, Doom 1, Doom (2016), Heretic, Witcher 3, Cyberpunk 2077, etc.

There is a lot of more old soundtracks that i remember, but the best for me is the Baldur´s Gate 2 soundtrack, i played that game when it came out like 1000 of times or more + the expansion, every track was perfect, the intro, the battle music, the music in the taverns, it makes you feel like you are in that world... omg now i want to play it again DAMN YOU GOG!!

Sonic Adventure 2 soundtrak by Crush40 has great pop/rock songs that are also neatly tied to the game's events and characters. "Live and Learn" will always remain in the hearts of those who fought the Finalboss.

I'm honestly surprised that noone mentioned Sonic music so far - not only the series has one of the most iconic soundrack, those songs are probably the only thing that was consistently good throughout the sereis. Even Sonic Forces soundtrack is pretty good.

The Witcher 3's OST. The whole soundtrack compliments the beautiful visuals so perfectly. It avoids the usual Hollywood style, extremely generic and non-memorable music and the whole thing feels like it belongs only to the Witcher's world. The fields of Skellige is especially a beautiful scene visually, but complimented by its music, it just made me simply stop in my tracks and slowly take both visuals and music all in. It was almost like I could even feel the chilly winds on my skin the second I closed my eyes. It's something I've never experienced before.

Priscilla's song was also on another level. The closed atmosphere of the pub/bar, the audience slowly paying more and more attention, all the different kinds of reactions from them, they all resonated with me and started tearing up by the end. Even the usually cold Geralt showed some rare emotions when he listened with his friend, Zoltan. It was like a little island in this cold, war-torn world, where human emotions still existed. This was only able to hit so hard because of the quality of the music, lyrics and singer.

And don't get me started on the different kinds battle music. The way it gets me pumped for fights, no matter how trivial the enemies are is just absolutely superb. I listen to the Witcher 3's OST even to this day and the music alone is a big reason why I want to do a second run after all these years. I could go on and on, but I really shouldn't.