My favorite piano composers are Scriabin and Debussy. Leo Ornstein wrote almost exclusively for the piano, and has some very impressive stuff, as well. Also, I'd look for Les hueres persanes by Charles Koechlin.
Messiaen was a genius and possibly the greatest composer of the 20th century. I particularly like Des Canyons Aux Étoiles, and his Quartet for the End of Time is very popular.
If you want sypmhonies, I reccommend Shostakovich. He also wrote a set of preludes and fugues as inspired by Bach, and it is a masterpiece.
Lili Boulanger wrote some very good choral music, and died hideously young.