Interesting question. All of our modern western music stems from western classical music. I believe that the fastest and most effective way to learn the fundamentals of triads, diatonic chords, progressions, harmony, voice-leading, and chord-scale relationships is through the study of traditional classical music.
You don't necessarily have to play classical music, although that would certainly reinforce certain ideas. Studying and listening to classical music should be enough to help you get a grasp on the basics. I say classical music because it adheres very closely (for the most part) to a strict set of principles and rules that lay the foundation for all music. First learn the rules. Then learn how to break them.
The next logical step in furthering your musical education would be jazz theory. Rock, blues, country, and folk etc. are all fantastic music genres, but they didn't break new ground musically like jazz did.
Music theory can become an extremely complex beast to tackle. Use it for what it's worth, but never let the music stop being music.
Cheers,
Mat
