21st Century, Jeremy Soule. Truly, there is some great classical music being composed for video game soundtracks.
Shostakovich is always a great 20th Century orchestral option - not as tonally adventurous as other contemporaries, but if you're at all a fan of the symphony from Beethoven through Mahler, then you should definitely check him out.
I have two Golijov operas. They are an acquired taste. I have not acquired it yet. Only in parts.All with significant output after 1950.
Classical:
Dmitry Shostakovich
Darius Milhaud
Leonard Bernstein
John Adams
Meredith Monk
Osvaldo Golijov
Witold Lutoslawski
Movie Music:
John Williams (duh!)
Nino Rota
James Horner
Randy Newman
Interesting piece - thanks for the hint. There are so many talented composers.George Rochberg. A serial composer who returned to tonality with his 1972 hit, String Quartet No. 3.