I was never that impressed by videogame soundtracks, just took them in as part of the whole experience and left them behind after finishing the game. Two things have stuck with me though:
The WarCraft II (base game) soundtrack is my Reference for what medieval war game music should sound like - still enjoy listening to it today.
Some of the '99 Unreal Tournament tracks are great music to do any fast-paced high-focus activities to, e.g.: