BTW, I somehow forgot to list Camille Saint-Saens as grossly underrated. He was not only another child prodigy capable of matching both Mozart and Mendelssohn, but also wrote some wonderful music, possibly as colorful as Tchaikovsky's and as thoughtful as Brahms. Yet he isn't even considered a major composer by himself.
Which leads me to think that, generally speaking, French and Russian composers tend to go underrated (think Berlioz, Saint-Säens, Faure, Scriabin, Kalinnikov) whilst Austro-German composers tend to go overrated (think Wagner, R. Strauss, Mahler, Bruckner, Beethoven, Mozart, Brahms). Perhaps Wagner did too much of a good job convincing everyone that German-speaking composers, provided that they weren't Jews, wrote works of more intellectual and philosophical depth (???) whilst non-Germans (including German-speaking Jews) wrote nothing but banality...
Also of note: overrated doesn't necessarily mean bad. Just sayin' ...