America's greatest?

Rgable nominated, rightfully, Steve Reich's Music for 18 Musicians for a place as "The best work of American classical music"

While i agree that Reich's work is probably the best thing to come out of the last half of the 20th century, i would also nominate some other works for consideration. I have mentioned it before, but Samuel Barber's Knoxville, Summer 1915 is truly an american masterpiece, Copland's name has already been tossed around, so no need to continue there.

(you can yell at me for this)... Philip Glass' Einstein on the Beach may not exactly be a masterpiece, but it is pivital, as is Terry Riley's In C, so i feel it would be wrong to leave these works out.

Now i can be sly. I have already stated that Stravinsky is America's greatest composer, so i must nominate his later semi-seralist ballet Agon (which was written after he became an american citizen). There's gotta be some more stuff im missing, but i just cant think of any more right now. Thoughts? suggestions?

EDIT: I agree with Michael that the premise of the post is a little ridiculous... but as long as no one takes it too seriously- i think its fine. Also, i think its very silly to think that we can all agree upon one piece as the example of america's greatest classical composition, so thats why i presented several "nominations".