My short answer answer to this question is:
top 5 underrated
1. Samuel Barber
2. Alexander Scriabin
3. Camille Saint Saens
4. Aram Khachaturian
5. Marjan Mozetich
Honorable Mentions: Vincent d'Indy, Edward Grieg, Aaron Copland, Eric Whitacre, Jules Massanet, Alberto Ginastera,
top 5 overrated
1. Mozart
2. Berlioz
3. Brahms
4. Schoenberg
5. Elgar
Honorable Mentions: Haydn, Vivaldi, Johann Strauss, Phillip Glass, Charles Ives, John Cage, George Crumb, John Corigliano
And I just want to mention my favorite newer composers who are still writing and producing and to my opinion don't get enough credit for what I believe to be continuing classical music in the write vein.
Those would be:
1. Adam Schoenberg
2. Michael Gandolfi
3. Christopher Theofanidis
4. John Adams
5. Jennifer Higdon
6. Robert Aldridge
7. Jonathan Leshnoff