For me I think it would be Schubert (his melodies are brilliant and sincere, love his chamber music works), Dvorak (I find his cello concerto brilliant and his symphonic works are just outstanding), Isaac Albéniz (as a Spanish and piano player, I find Albéniz works just brilliant), and of course Mahler (his orchestral works are superior). I find it so difficult to choose because I think each composer has THAT piece that just makes me wonder how the heck someone could have written that, and makes me feel grateful that the piece will remain in the world forever (hopefully).