Lineage II

Probably it's generally because of nostalgia about thousands happy spent hours, all those nights and days. But how can you don't have goosebumps when you hear Dion Theme? Even aside from nostalgia Ncsoft did a gorgeous work in that area. Just like in HOMM III every city got it's own unique music theme, what's more, there are plenty of atmospheric battle OSTs, location music, etc.

P.S. HOMM III music is just out of any contests, it's simply a masterpiece.

HOMM II - Magnificent Field (Grassland Theme)

Heroes of Mightand Magic II's Magnificient Field is so memorable to me because my childhood best friend introduced me to the HOMM series when I was 8 years old, and growing up we started with the original game and I visited him every week and every time we played a HOMM game through the afternoon. First HOMM, then HOMM II, then HOMM III then HOMM IV. The Grassland Theme is one of the most memorable themes in HOMM II and we played so many hours of HOMM II. Of all the games, I think we played it most. We hardly played any other game for the several years we saw each other every week.

Definitely the Secret of Monkey Island, because the music itself creates a whole new world where swashbuckling, grog-drinking and pillaging are key!

I'm still partial to Heroes of Might & Magic 2 to this day. Heroes 3 was pretty grandiose in its music but Heroes 2 made the environment feel alive. It will always hold a special place in my heart.

I do not leave the music on while playing most of my 4,000+ games. However, Diablo's one and two always compelled me to break that rule. It works in a wonderfully understated way, always supporting the experience, never distracting.

Furi

A soundtrack that carries the whole of the games atmosphere of adrenaline pumping action, while still differentiating between it´s unique characters distinct themes and feel, keeping you in the moment as well as pushing you forward. Phenomenal

GTA Vice city. Probably nostalgia as I was a teen in the 80's but, man i could just cruise round all day listening to that soundtrack.

Chrono triggers soundtrack was that for me, like I listened to that soundtrack over and over while I was studying. I would use that to help me focus but also it kinda backfired on me and distracted me because some of the songs are so good I'd just bop to them for a bit.

Sid Meier's Civilization IV. Baba Yetu has to be the best game soundtrack (well, main menu theme) there is. It's the only game where you almost don't want to play because the song is so good! Christopher Tin is a genius!

There are many games with beautiful soundtracks / music / sound effects, besides HoM & M3 which I love because the peace it conveys, one of the first games I loved for music was the first Command & Conquer. It gives me the feeling of being in the action, outstanding!

of course zelda ocarina of time. my first video game experience after a long game pause from c64 times. with 3d grafix absolute lovely tunes which guide u until today. of ocurse iv the original soundtrack by nintendo but on cd <3

Favorite of all:
Need for Speed ​​3 - Hot Pursuit: Pi
downloads.khinsider.com/game-soundtracks/album/need-for-speed-3-hot-pursuit-pc-rip/08%2520pi.mp3
Many times I just let the game start menu sound in the background and deal with something else, or left the car selection menu on the screen, let that blue Corvette spin on the stage! At the age of twenty, with empty pockets and a fresh driver's license, it was good to dream about it ... And of course I spent a lot of hours playing.

I just have to speak about some more memorable game music:

The Command & Conquer - Act on Instinct was also a great experience!
youtube.com/watch?v=n4AUY-v1nsE&list=PLQGm88aU9WILa4QizU3BOvBVAz1fxRNwh
I pushed the game so much anno that the melody burned deep into my cerebral cortex.

Unreal 1. Intro or Castle FlyBy.
youtube.com/watch?v=26I-Pw-yPJ4
Another catchy tune! Let’s say I watched and listened a little while admiring the ability of my brand new Woodoo II graphics accelerator card. Back then, this picture quality was really cool! Anyway, good music was played throughout the game, and it was the first game on the market (as far as I know) where the style of music changed dynamically according to the gameplay.

Panzer General 3 Scorched Earth - Battle music:
youtube.com/watch?v=tBkLyz2TYkQ
Well, it also burned into my brain properly! I spent a lot of nights with this game too!

Diablo I - Tristram village
youtube.com/watch?v=1cH8ZwL3A7Q
My God ...I even bought a guitar, but it's definitely not a beginner tune.

Shogun - Total War 1, background music when moving units on the field.
youtube.com/watch?v=4IU7ia0XIGg
Although the instrumentation is simple, but so good!

Bonus:
C64 - Falcon Patrol II, intro
youtube.com/watch?v=RvY7jGlQPCk
My first encounter with computer games at the age of 12 ...
How could you forget the first time? :-)

Well some of the first games with memorable soundtracks were the most memorable to me. Among them is Heroes of Might and Magic 2 with its Opera sounds as well as Quest for Glory 1 - quite different games as a feel and music maybe but these two are among the most memorable to me.

Heroes of Might and Magic III

Although I'm a simpleton when it comes to music, even my pathetic ears know it's a masterpiece. The town themes are especially great, flawlessly capturing the feel of each town, making the players feel at home in every Castle, Necropolis or Fortress.

Transport Tycoon Deluxe
Relaxing jazz music wonderfully supports playabiliti.

Warcraft II
Whole game has an amazing music and units voices.

Age of Empires I
Similar reasons like Warcraft II, bud with different kind of music completing atmosphere ancient nations.