u should definitely get some rachmaninoff or rachmaninov (whichever way u wanna spell it). try rach's 3rd piano concerto. and his rhapsody on the theme of paganini is just magnificent. get some of his preludes of piano as well. the no.5 in g minor is just too good.
if u can, get the whole set of preludes and fugues of bachs well tempered klavier book 1.
Brahms Rhapsody in G minor and B minor are good. His piano writing isnt amazing, but very nice. I would call it "orchestral" style. his variations of the paganini caprice are also mind blowing.
I am not particularly aquainted with chamber music, but i know that schubert and mozart have particularly good stuff in that area.
if u like classical guitar music, then get the guitar concerto's by mauro giuliani. and some solo guitar works by Fernando Sor and Francisco Taregga, and Mangore.
For piano. chopin and liszt are a must. recommend listening to anything by them.