Underrated:
1. Kalevi Aho. Probably the finest Finnish symphonic composer ever. Yet his 16 symphonies are not widely known. Of course not all of them are first-class, but numbers 1, 5, 7, 9, 10 and 15 are truly awesome.
2. Camille Saint Saens: he is much better composer than he is usually given credit for. His piano concertos and symphonies are outstanding. His clarinet sonata is such a beauty! He is not just a composer of Animal Carnival, Dance Macabre and Samson and Dallila.
3. Gustav Holst: He is known only for Planets but he has composed lots of exciting music, especially vocal. And his music from opera The perfect fool is intriguing.
4. Levi Maadetoja: his symphonies can easily match the quality of Sibelius, especially no. 2.
5. Lucijan Marija Skerjanc: I purposedly mention this Slovene composer since he is much underrated even in my country. But his symphonies no. 4 and 5 plus some other compositions (string quartets no 3, 4 and 5) should be world-known.
Overrated:
1. Anton Bruckner: I give him credit for symphony no. 4, otherwise a boring, boring composer. Sorry.
2. John Cage: he had some intriguing ideas but he was never supposed to become a guru of modern music since he never really showed serious artistic tendencies.
3. Giuseppe Verdi and Giacomo Puccini: while they are excellent opera composers they are overdosed.
4. Gustav Mahler: funny, just as Bruckner, he was also the luckiest with his fourth symphony. The no. 1 is also ok, but he has made other symphonies way way too long. A smell of self-exhibitionism is too obvious.
5. Frederic Chopin: I am beginning to avoid concerts where is music is present. I am surprised the pianists are still so attached to him. Why not trying something else?