Because the other thread by that computer simulation xumeng just mentioned Mussorgsky in favor of the less original but more successful Rimsky Korsakov, I have to post something about the greatest of all Russian composers. He may have been messed up in his private existence, which led to problems with his music, but he established Russia as a pure musical entity infinitely more than Tchaikovsky who was kowtowing to Western European traditions and therefore internationally famous at the time that Mussorgsky was unknown. However, the songs and operas of Mussorgsky, not to mention the spectacular showpieces of Pictures at an Exhibiton and Night on Bald Mountain, incorporated the slavic motifs and folksong he grew up with in an intensely personal romantic style that is far above the others of the so-called Mighty Five. There is an excellent bio of him recently published by David Brown that is far moe complete than previous ones.
btw, his original orchestration of Night on Bald Mountain - which was used by Stokowski - was owned by Gregorovich Stanislavsky (Gregory Stone) who created the Reno Philharmonic here in Nevada cowboy country.