A few more composers:

John Adams.  I love his opera Nixon in China.

Thomas Ades.  Try Tevot, or the Violin Concerto.

Max Richter.   The Four Seasons Recomposed.

Frederic Rzewski.  Marc-Andre Hamelin's recording of his "The People United" is astonishing.

And on the topic of Philip Glass, I prefer many of his later, more sophisticated music in comparison to the early, more minimalistic works.  His operas Orphee (based on the Cocteau film) and Kepler (it rocks!) are particular favorites.  Also the Eighth Symphony, and the first Violin Concerto.

A little off-topic, but I was pleased to see a reference above to Arrigo Boito's great opera on the Faust legend, entitled Mefistofele.  

It's not contemporary.  But it is one of the greatest operas, musically stunning, sweepingly melodic, richly orchestrated  and lyrically fascinating (based on Goethe).  In my view, it far surpasses Gounod's Faust.  I strongly recommend the recording with Caesar Siepi and Renata Tebaldi.  Excellent sound, btw, even though recorded in the 1950s.

If you like Verdi (Boito was a librettist for Verdi) or Puccini, Boito's Mefistofele is a must.