Just listen by yourself! I picked just some of my most favorite tracks and even one Medley of an entire game’s music. I even own a 300 games large music archive, and it’s hard to choose the very best. So I took some game soundtracks I’ve listen to right now the most.
Descent
Atmospheric Lab
Command and Conquer: Tiberian Dawn
Just do it!
Metal Gear 2: Solid Snake
Theme of Snake & Zanzibar Breeze
System Shock 2
The Coolant Tubes
Sweet Home
Medley
A3D's Let's Plays 
Nice selection. I would add Quake 1 and Quake 2 :)
Turrican 2, The Last Ninja 1+2 both Stage 1 ^^