Its been about 9 months since i posted on EA, but since this thread is still alive I will add a bit to the info. (thanks mr. nicchitta)
Alan Hovhannes is becoming more and more important in my life. He really is one of the best 20th century composers, following no ones tune but his own.
George Antheil - american composer. His Ballet Mechanique is a must have. Home stereo speakers cold explode at this piece for four pianos and a shitload of percussion. Its one of those pieces that make you go "holy shit!". the rest of his work is good to great, with a big nod to the neo-clasisists.
Benjamin Britten - ok, i used to hate this guy to due some of his cheezy orchestral stuff, but he really has some very nice chamber stuff. Its not earth shatering original or ultra modern, but it is sincere and the emotions expressed can have quite a whallup.
Robert Casadesus - (French) His chamber stuff is ultra beautiful. Not modern by any great means, but is sure pretty and just sounds flat out great. Dude is super french, so you know its gonna have flowers popping out all over the place. If you are looking for someone like a bit more modern Debussy, he is your man.
http://en.wikipedia.org/wiki/Robert_Casadesus
Henry Gorecki - (polish) dark as dark gets. Goth folks would buy stock in razer blades and bath water after hearing him. Fantastic stuff, but super depressing. He is almost like a more accessible but 10X more dreary version of schnittke. Emotional stuff that is super real. The Kronos versions of his string quartets are quite good and very easy to come by.
Lou Harrison - (american) buddies with Cage and Cowell, and all of the who's who of the 40's-60's avant garde. A lot of his work is very percussion orientated, and most of it quite accessible. Great speaker fodder. You really cant go wrong with most of his stuff, and due to the use of ethnic percussion instruments, a lot of folks that might be turned off by modern classical might even dig him.
Roy Harris - (american) in the Ives, Cowell, Piston, Copeland family, his stuff is quite nice but not super modern sounding. He has his atonal minutes, but is not way out. While he did not see himself as neoclassical, he pretty much ends up being an american take on the idea.
David Diamond - (american) His string quartets might possibly be the best that any US composer has come up with. They are almost a more lyrical and american version of the Bartok String Quartets. For anybody that is a fan of string quartets, these are a must have. They are as good as it gets. Super intense and over the top emotional, they transcend any genre or style. 5 stars out of the 4 possible.
Ahmed Adnan Saygun - (Turkish) Great stuff. Wikepedia says it better than i can
---Ahmet Adnan Saygun is acknowledged as one of the most important 20th century composers in Turkish music history. He is a master of the neo-classical form His works are rooted in the western musical practice yet they incorporate traditional Turkish folk songs and culture. He usually adds this folk element by picking one note out of the scale and woves a melody around it using a new Turkish mode. His extensive repertoire include five symphonies, five operas, two piano concertos, various concertos, and a wide range of chamber and choral works. The London Times called him “the grand old man of Turkish music, who was to his country what Sibelius is to Finland, what de Falla is to Spain, and what Bartók is to Hungary.” As Saygun was growing up in Turkey he witnessed radical changes in his country’s politics and culture as Mustafa Kemal Ataturk had replaced The Ottoman Empire, -which had ruled for nearly 600 years-, with a new secular republic based on Western models and traditions. As Atatürk had created a new cultural identity for his people and newly found nation, Saygun had found his role to develop what Atatürk had only begun.
Elizabeth Maconchy (English) Her string quartets are what her reputation is based on, and rightfully so. They are some of the best English SQ's, with just the right amount of modern to make them quite a worthwhile listen. They are neoclassical in sound with a bit of Bartok to them.
Reynaldo Hahn (French) One of those great composers who fell through the historical cracks. Ultra lovely stuff from the apron strings of Debussy and Ravel. Early 20th century stuff that is more modern than it sounds. Speaker butter, and fans of Debussy and Satie will feel like they have uncovered the missing nugget of early 20th century French music. Really pretty, like that cute girl at the coffee shop who dresses just right, and who is almost nice enough to think you have a chance, even though you don't, but it doesn't matter because she smells so sweet.
Well, thats enough for right now. Next time around i will touch on Xenakis, Stockhausen, and some others.