I don't know anything about overrated or overplayed, but Debussy, Shostakovich, and Schoenberg do nothing for me. I absolutely adore Shostakovich's Second Piano Concerto, but that's about it. Debussy wrote some charming pieces for the orchestra.
I definitely think that any and every composer can be argued to be underrated, or at least, underappreciated. Many Soviet composers, such as Protopopov, Feinberg, Roslavets, and Rakov are only a few out of the hundreds of forgotten geniuses. And there are Medtner, Scriabin, and Kabalevsky, who don't tend to be the first choices -- or the second, third... -- for anyone who ventures the literature of classical music.
I personally feel that Mozart is one of the most underappreciated composers whilst being one of the most overplayed. Not nearly enough people appreciate his piano concertos, especially his 14th, 17th, 19th, and 26th.. they all contain some of the most supreme moments in the entirety of music. And of course, there are breathtakingly poignant 22nd, 23rd, and 27th (which has to be my personal favorite by Mozart). And his early gems are not to be missed -- 4th, 8th, and 12th. As Emmanuel Ax had once said, it is immensely difficult to say anything nice about Mozart, because they are all stating the obvious.
Most composers who are now known for THE work -- Faure and his divine requiem, Moszkowski with his Caprice Espagnol, and Respighi with his Roman Trilogy. I'm yet to listen to Respighi's Piano Concerto and Modo Misolidio, which deserve a new category of revitalization. Respighi's Piano Concerto in A Minor arguably outshades both Schumann's and Grieg's, yet has only a fraction of the popularity of the two famous staples of piano concertos. To be honest, Grieg's piano concerto is pretty boring.
And more people should play Faure's chamber music and piano music. His barcarolles, Ballade, and Piano Trios are to die for.
And last, but not least, Moszkowski's Piano Concerto belongs to one of my favorite piano concertos of all time. It inspires me, makes me tear up, and makes me elated in a way that Brahms's 2nd Piano concerto can only dream of.