One of the most underrated composers in the industry has to be Yuzo Koshiro. The main composer for an equally underrated series, Etrian Odyssey. People are often off put by its art style, as it betrays how intelligent the games really are.
He has also composed for many other games such as Streets of Rage, and Act Raiser. He composed some of the best "MIDI style" music in the industry, as well as remastered some of those same tracks in a proper arrangement.
He isn't pigeon holed into a single genre, he composes many different styles of music ranging from Jazz to classical, from electronic to rock/metal. He uses whatever genre he needs to really set the tone of a scene and to bring life to a game.
I'm going to submit his newer stuff that showcases his incredible range and talent as a composer.