Joshua came up with a fine list, except that almost all of it is from the last 30 years and almost all of it is in, hmm, I'm not going to be able to come up with a good phrase to cover all of those guys 'n gals. Without having heard it, I think I'd pitch the Kernis symphony in favor of something by Saariaho, say, Graal Theatre, even though that would make his list even more Finn-heavy, and I'd swap Magnus Lindberg's Seht die Sonne for the composer's fantastic Clarinet Concerto.
I'm kinda shocked that there's nothing on his list by Shostakovich.
And, really, you can't survey the orchestral music of the last 50 years without throwing in something big by a high modernist or two. Can't we get one of Carter's big essays out there? Some Stockhausen? Birtwistle? Wuorinen?