Pfft. This is easy - Shenmue's music (composed by the amazing Yuzo Koshiro) is the best. Yes, it's a fact, not an opinion. Seriously. Everyone
will agree with me. By force, if necessary.
Outside of that, I love the soundtracks to the Rockman / Rockman X series, the Guilty Gear series' soundtracks are excellent, and of course Koji Kondo's music in the Legend of Zelda series is always awesome.