The MechWarrior 2soundtrack. It's probably among the most memorable video game soundtracks ever. It makes even the most mundane activity in the game seem epic. Even listening to them outside the game, they sound like they should be played while something epic happens.
Wednesday 7 from Mechwarrior 4: Vengeance is up there too. In fact, a lot of the Mech' series music is good, whether it's from #4 Mercs, either of the 'Commander games, or the other 'Warrior games. Also in #4 Mercs; Behemoths for the Heavy/Assaults and Skittersprint/Floodgate for the Lights and fast Mediums.