The best pieces from the 21st century are undoubtedly written by John Carey. He's amazing! Seriously though, I'm not really familiar with many 21st century composers. Leo Ornstein lived into the 21st century, but not much of it. Of course there are composers like Penderecki, Rzewski, and Finnissy that are still alive today, but I personally think we should wait for another 10-15 years before deciding on the "best pieces of the 21st century".