Command & Conqer, the game that kickstarted the RTS genre, is not only a great game but has an amazing soundtrack by Frank Klepacki as well.
You can listen to all tracks or buy the OST (which, sadly, does not contain the full soundtrack) here: Frank Klepacki homepage.
I installed the C&C DOS version in order to make the homepage video, and it's still holds up in playability, with a soundtrack that fits the action incredibly well.
If you want to know about the origins and inspiration of the OST, then there's already some good information available on the C&C Wiki.
The C&C soundtrack mixes rock with electronic music to create an atmospheric background; one time you will be quietly harvesting resources with a mellow electronic song in the background, another time you will destroy a base blasting pumping rock music.
Overall the compositions and melodies are great, and it's hard to top this soundtrack in terms of different sounds and moods. Luckily the later released Red Alert soundtrack is a worthy follow-up, and will be added on this website as well.