Harry Partch, who not only was a prolific composer but also developed his own instruments and microtonal scales should also be included in any list of the "underappreciateds".
More than one here, so can't be "most" but perhaps, "very"?... Edgar Varese George Crumb
While I do not have first hand experience of their work, from doing a little reading on "minimalism" as it pertains to music, these two names deserve consideration: La Monte Young Henning Christiansen
If by "underrated" we simply mean deserving more acclaim than we think has been afforded so far, I would say Prokofiev. But maybe I'm just "out of the loop."
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