I'm not sure if the Metal Slug franchise is still well-known (or ever especially popular to begin with), but when I hear the word "soundtrack," Metal Slug 5 tends to come into my mind. It's not exactly the best soundtrack or the most diverse one, but I could never go wrong listening to it. I would describe the majority of the soundtrack to be this "middle ground" between hard rock and speed metal with a bit of techno sprinkled onto it.
http://downloads.khinsider.com/game-soundtracks/album/metal-slug-5-original-soundtrack (some of the soundtrack is missing here, unfortunately).
The more notable ones are Zoom Down, Speeder, and Final Attack (granted, this theme is in almost every Metal Slug game).
The Youtube links are for those wary about the website. I doubt there's viruses, but I have virus protection on, so I wouldn't know anyway.
Zoom Down
Speeder
Final Attack