Bartók, Bernstein, Cage, Babbit, Gershwin, Varese, Messiean, Stockhausen, Takemitsu.
See: Wikipedia List
** From A History of Western Music: "Through external circumstances and the force of his own genius he transformed this heritage and became the source of much that was characteristic of the Romantic period. But he himself is neither Classic nor Romantic; he is Beethoven, and his figure towers like a colossus astride the two centuries." [Donald Jay Grout. (3rd Edition with Claude V. Palisca.) W. W. Norton & Co., Inc. New York. 1973. p. 521.
Newest ed. of the Grout, 9th ed., Burkholder (includes ebook and digital recordings).