The thing is, that while I am in complete agreement with a lot of the pieces, I don't think that every piece, in order to be great, has to be so difficult, both for the performers and listeners, all the time. I think that aesthetic greatness has more criteria: expressive intensity, humanity, breadth of expression and so on. Nearly all of Boulez' choices, including his music, are harmonically dissonant, dynamically extreme, rhythmically jagged and with pointillist texture. I don't think all great music must be like that, even in the 20th century. But that being said, yes, these ten works are classics of 20th century music.
I think I will make up my own list so we can argue about it. Some pieces I would consider including would be something by Steve Reich, something by Sibelius (if Boulez can include Mahler, I think I can include Sibelius), and something by Shostakovich.
But there is no arguing about this piece, which would easily make my list as it did Boulez'. Here is Boulez conducting the Music for Strings, Percussion and Celesta of Bela Bartók played by the Vienna Philharmonic in a live concert in 2007: