My top 5 picks: In no particular order... (except Herzog, man I love this one)
Herzog Zwei, Genesis -- No doubt about this one. Goddamn, I can't find a single bad track from this game. And the tunes are soooo spot on to the game environment. I'd pay good money to buy a soundtrack of this game, especially this
Ys I&II, TurboCD -- Awesome, awesome tunes again. Like above, I think they did a good job in matching the setting to the environment, though some disagree (some didn't think the guitar jams didn't quite fit). Nevertheless, awesome. My favorite's the tune between the two chapters when Adol gets transported into the land of Ys.
Actraiser, SNES -- Made really good use of the SNES music chip -- These tunes are so well suited to the SNES that wouldn't have quite sounded right on the PCE or the MD chip. It's so deep, and feels like an orchestra's playing this one out.
Streets of Rage, Genesis. Rad techno tunes, and its successors never quite matched up to the originals when it came to the music.
Lords of Thunder, TurboSCD -- Again, not to everyone's taste, but I do love those guitar jams.
Honorable Mentions:
Thunder Force III, Devil's Crush, Double Dragon (arcade, nes), Legend of Zelda