They're also incredible fun to sing - virtually all his pieces have 3rd bass lines down in the gutteral depths of the range, and it's the same up the higher end. They give singers a chance to sing in registers they don't necessarily hit :P

You're almost there buddy. Ban-hammer is awaiting. :)

Another important composer: Samuel Barber. These are the famous Adagio for Strings (1936) and Violin Concerto op.14 (1939):

Gyorgi Ligeti is probably by far my favorite post 50s composer. An original, genius, madman with a sense of humor:

As it's defined here I think it's way too broad. Anything from over 20 years ago I would struggle to label as modern in the usual sense.

Even now of course there is a wide variety of things, often crossing over with musicians from popular music (from jazz, 'rock', world music, electronica etc).