Most underrated composer of 20th Century?

I am a big believer in the concept of Yin and Yang.

THere are many things in nature that are both beautiful and dangerous at the same time. PEople learn to appreciate the beauty while avoiding the danger. Many might walk up to teh edge of a cliff for the scenic overview, but they also know not to jump off. Its part of survival of the fittest, natural selection, etc. Homo Sapiens have gotten to where they are today mainly by using their brains. If that cahnges, I have a gut feeling that rock music will have little to do with it.

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."

Conlon Nancarro! Works for player piano! I have a multi - CD set of these! Not one person in a million remembers him!