Hmm, let's see... in very rough, mostly interchangeable preferential order:
Rachmaninov
Shostakovich
Scriabin
Stravinsky
Prokofiev
Schnittke
Myaskovsky
Tchaikovsky
Mussorgsky
Medtner
There's quite a few major Russian composers to whom I've scarcely given much of a chance, including most of the famous Mighty Handful guys, etc., but those I named are solid favorites. There's definitely a chance that I may be forgetting people, too.