Anything by Sorabji! He is one of the most underrated composers there are! I'd say his Fantasia Espagnole is a neglected masterpiece that most people could appreciate if they actually heard it!
And, one of my favorite composers, Leopold Godowsky. I have yet to hear a piece by Godowsky that I didn't like! I suggest to everyone that they check out my topic on Godowsky (back when I was still Ludwig Van Rachabji) and listen to some of the samples I provided. His music is wonderful!
Penderecki is another one. I am not usually a fan of atonal/avant-garde music, but Penderecki's music is so exciting. I love his Canticum Canticorum Salomonis.