If you asked, I could tell you my favorite musical artist without hesitation. I could tell you my favorite album from said artist without much deliberation. I could even tell you my favorite instrumental album. However, I don't think it's possible for me to pick my favorite game soundtrack. From as far back to Kirby's Adventure to as recent as Doom (2016), from small indies like Hotline Miami to massive blockbusters like Halo 3, there are just too damn many high quality, well composed video game soundtracks out there from me to pick a favorite.

Hotline Miami
Lost Eden
Machinarium
Treasure Adventure Game
Within a Deep Forest

Two Worlds.
I also like the game itself, which is very unloved though. but if game is just nice, soundtrack is nearly amazing. written by Harold Faltermeyer and performed by mysterious band Amber Moon known only for this only work. and I just love this [url=https://www.youtube.com/watch?v=_3dPSlV6Jf4 ]theme song[/url].

Pokey (Atari 8-bit):
Lemmings
Hivmania
Helicops
​Draconus
Rebound (HB)
Black Lamp
Tempest Xtreem
Xirius Defect
Zybex
Jet Set Willy
​
SMS/GG:
Alien 3 (SMS/GG)
​The Flash (SMS)
Devilish (GG)
Streets of Rage
Power Strike 2 (GG)
The Smurfs (SMS/GG)
Tintin in Tibet (SMS/GG)
Road Rash (SMS)
Arena: Maze of Death (GG)
Sonic the Hedgehog (SMS)
​​
Links:
http://minirevver.weebly.com/pokey-atari-8-bit.html
http://minirevver.weebly.com/sn76489-sms--gg-tandy-1000.html

dune 2

doom 2

https://www.youtube.com/watch?v=0U-3XqcTcIk
https://www.youtube.com/watch?v=gsvvIut19uA

Mega Drive/Genesis:
Thunder Force IV
Ristar
Master of Monsters
Streets of Rage 2
Sonic 3D Blast
Gauntlet IV
Alien Soldier
Mega Turrican
Vapor Trail
Dragon Slayer: The Legend of Heroes

Amiga:
Turrican II (AMI)
Cannon Fodder (AMI)
Fury of the Furries (AMI)
Lotus 2 (AMI)
Utopia (AMI)
Space Hulk (AMI)
Pinball Fantasies (AMI)
Pinball Dreams (AMI)
Zool (AMI)
The First Samurai (AMI)

Links:
http://minirevver.weebly.com/ym2612opn2-mdgen-w-sn76489-mcd-32x-fm-towns.html
http://minirevver.weebly.com/paula-amiga.html

I got a few of them. There's the soundtracks for Doom 4, Brutal Doom, and the PS1 Doom port. Chrono Cross, Shantae (both Half Genie Hero and The Pirate's Curse), and music from both Starcraft and Starcraft 2.

Me, I have fond memories of the music in the Redneck Rampage series of games.

Hillbilly meets Bluegrass meets Rock meets Heavy

King's Quest I+II's death theme

Aside from that:

Anno 1701
Industry Giant 2
Majesty 2
Simon the Sorcerer 1
To The Moon
Transport Fever
Tropico 1

LotRO
Lord of the Rings Online
This game has an awesome soundtrack. If you are having a party in the Shire or walking into the Golden Wood of Lothlorien or learning to ride a horse in Rohan, the music in this game will carry you deeper into the world of Middle-earth. :)

still always:

Interstate '76

this

Battle for Wesnoth.

I kind of like the Deus Ex and HoMM 3 soundtracks as well.