It would be hard to nail down a favorite, as my childhood was spent playing many classics. Age of Mytholgy's soundtrack did a wonderful job of differentiating the playable cultures, while also ramping up the battle music when it detected a major offensive move by the player. Dragon Age: Origins has a score that I can recognize within a few notes. Then, of course, there's the beloved Heroes of Might and Magic series. I am just old enough to remember what life was like when there were only 2 HoM&M titles. Too young to have appreciated the music purely for its own sake, perhaps, but in retrospect, I'm still quite floored by the quality, especially that of the 3rd game.

I enjoyed the music from Dark Wizard (Sega CD) to listen to not only at home but also while driving.
Back in those days, a good soundtrack was hard to find. Dark Wizard stood out and is still one of my
favorites if not my number one.

I love the soundtrack to Elder Scrolls III: Morrowind. The tracks just fit their locations perfectly and it was ahead of its time in many ways. It also hits my personal bias towards nostalgia.

The soundtrack to Civ IV.

The track Baba Yetu by Christopher Tin says it all.

Metroid Prime on GC sparks major vibes and slaps as hard as a mule-kick.

My absolute favorite game album of all-time is "Chrono Trigger: The Brink of Time."
Growing up as a youth, Chrono Trigger was my favorite SNES game, and I must of listened to all the different reditions of the Chrono Trigger libray, but especially the arranged "Brink of Time." I have to have listened to it over 100 times beginning to end.

1. Goldeneye 007 (1997)
Goldeneye 007 (1997) for me. Hearing this soundtrack on the N64 while playing single player or with friends in a deathmatch game (4 controllers on one TV pre-internet) is pure nostalgia. I still find myself listening to higher quality versions of this soundtrack today, but back in the 90's this 2 hour soundtrack created a truly immersive gaming experience. Every track was on point with nothing feeling like filler. This game and soundtrack did not just copy the movie, it added its own world locations with region appropriate tracks using local sounding instruments. Who could forget the track “Facility”. A feat that was not successfully recreated with later Bond games, and stands out among the best in gaming history.

2. Umineko When They Cry (2007)
In close second, if visual novels could count as games I cannot forget to mention 07th Expansion's Umineko When They Cry. Umineko has a soundtrack that I think is among the best in quantity, musical styles, and quality of music. It features 13 hours of music with over 210 tracks by dozens of individual artists. Even with this length, amazingly songs like Worldend Dominator and Dreamenddischarger really stand out among everything else available today. Thank you Ryukishi07 for creating this epic VN that allowed for such outstanding music to be brought to the world.

Resident Evil 2 (1998) is probably my favorite, as far as RE soundtracks go. Tracks like The Marshalling Yard (The Second Half) is one of my all time favorites, with some nice variations and changes happening throughout. Resident Evil 3: Nemesis (1999) had a good soundtrack, I know the focus was more horror and ambient oriented so all of the tracks may not appeal to everyone but I still thought it was cool.

Oh, no question about it! The Elder Scrolls IV: Oblivion has the most memorable game soundtrack for me, period. It's absolutely gorgeous and atmospheric, ethereal yet comforting.

The droning strings and peaceful piano of Auriel's Ascension stirs my heart in ways I can't explain, bringing to memory all the beautiful moments when I'd just let my character sit or stand somewhere to take in the views. It particularly reminds me of when I'd just have my character jump to the roofs of Bliss in New Sheoth (in the Shivering Isles) and watch in awe as the night-time pink spore clouds gently drifted overhead.

Then, there's the ambient outro of King and Country which reminds me of intentionally walking my horse around the valleys of the Nibenay Basin during the day, enjoying the gentle sunlight and sound of the leaves rustling in the wind.

Or maybe All's Well, which conjures images of my character sitting inside an inn in Anvil, enjoying the (sometimes comedic) banter between the NPCs as the gorgeous harp and flute serenade my ears.

Oh, and there's also great combat music, such as Defending the Gate which appropriately feels like the soundtrack to a fierce defense. Sure, it sometimes feels a little less than right when battling a low-end enemy but when you hear it in the more epic battles later on, well... in the words of Todd Howard, "it just works."

And of course, there's the absolutely amazing theme track, Reign of the Septims. While Skyrim's menu music has something special going for it, this rendition of the TES theme is just too memorable for me. It really does feel like the music of the final days of the Septim Empire: full of glory and prosperity that's all precariously teetering on the brink of total destruction.

All of this game's incredible soundtrack comes together to solidify why Oblivion is a top 3 game for me. It just helped sell the world to me and thanks to it, all of my great memories of the game are always just one listen away.

My favorite is probably Ave Maria from Hitman.
Other than that
Three stand out above every other as being battle themes:
1)Golem fight soundtrack from the Witcher 1( can't find its name)
2) Tower Encounter from the Prince Of Persia Warrior Within
3) The Only thing they fear is you from Doom

Other than that: The Assassin's Creed Black flag theme, The Halo theme, Song for the Devil by Rolling Stones from Black Ops 1, GOW OST

To The Moon soundtrack is very emotional and memorable.

Definitely Monkey Island series. So Manny good memories waking up Sunday mornings and just playing all day.

CIV 4 - both the game as well as the soundtrack (Baba Yetu) are fantastic. The main theme gives the game a monumental feel, love it.

Whole HoMM series!!! (Academy theme from HoMM5, Grass theme from HoMM4, every single audio file from HoMM3...)

Heroes 3 and the first generation of pokemon games :D