Sgoilear,
Check
this thread out. It is a discussion about a lot of what you are mentioning. Roachboy and I wrote a lot of stuff there that I think you'll find relevant.
Part of the difficulty with seeing great composers of the second half of the 20th century is that there were so many great ones in the first half that come to mind. 1900-1950 was actually a tremendously fertile time for classical music. Strauss, Mahler, Stravinsky, Prokofiev, Bartok, Shostakovich, Berg, Shoenberg, Rachmaninov, Ravel, Hindemith, Webern, Copland, Ives, Puccini, Debussy, Sibelius, Kodaly, Vaughn Williams... That's an abundance of riches and I haven't even stretched to find name yet. Consider that someone born in 1950 is only 55 today - not too old to still write masterpieces.
As to the "greats" of the last 50 years, I think there are some. Poppinjay is right on with John Adams - I'm a HUGE fan. Check out Grand Pianola Music. I think Adams has really found a voice for modern America (just think about listening to his stuff while driving cross-country).
Steve Reich, Philip Glass, Christopher Rouse, and Arvo Part are all great as well. I like some Penderecki and Gorecki, and some of their stuff turns me off...
There is some truth to what ratbastid and WillyPete wrote as well - there is some great music in the film world, though the need to be profit-friendly can be somewhat constrictive. I've met Danny Elfman and he is quite talented... I heard his Concerto for Two Pianos last year. Frankly, it isn't as good as his film stuff. Of course, that's only one piece on one day, so take that with a grain of salt. John Williams wrote a tuba concerto that is pretty decent (not that there is a ton of competition there).
Seriously, check out the thread I linked.