Underrated:
André Caplet: the one who orchestrated Le Martyre de Saint-Sébastien from Debussy. He is also the author of one of the most beautiful mass ever composed in my opinion (excerpt of one movement here , absolutely stunning :
https://www.youtube.com/watch?v=Iavb5mnQl9Y
Modest Moussorgsky : Still don't get the recognition he deserve in my opinion.
Charles Ives, still very much underatted and underperformed in Europe.
Charles Koechlin, the author of one of the best orchestration book ever written in French (even better than the one written by Berlioz in my opinion). Koechlin was also a compositor contemporary of Debussy and Ravel, and he was indeed a great orchestrator.
Gavriil Popov: Great soviet composer, have composed a great first symphony. A little bit in the shadow of Shostakovitch.
Overrated:
Wagner: Historically, obviously an important composer. But I just can't stand a Wagner Opera. I don't like the way he wrote for voice.
Boulez. Great Conductor no doubt. But I don't think his work as a composer will really stand the test of time. It's my humble opinion of course.
Britten. A bit bland in my opinion.
Vivaldi.
Verdi.