Who is the most underrated classical composer of all time?

Clara Schumann for one.

Your question calls for an opinion, not a fact. You will probably get several different responses.

Clementi , Alkan and John Field

Since this is an unanswerable question, I am just going to suggest Obukhov. He wrote all his music in his OWN notational system (which should satisfy the “I don’t wanna learn no stink in’ music theory!” people). He wrote it in his own blood (hello goths), while dressed in fake religious robes (hiyah, religious loonies).  The music itself was crap - good morning, Taylor Swift and Katy Perry fans - yet pretentious enough to attract the snobbiest of “who is better - Mozart or Beethoven?" dabblers.  To listen correctly, you will need lots of candles - and a small theremin, enclosed in an Orthodox Mass cross and globe.

Vassily Kalinnikov, Sergei Tanejev. and a host of underplayed Russian composers.

Moritz Moszkowski is just one.