John Zorn is a seemingly inexhaustible source of great, inventive modern, often quasi-classical music, and the great majority of the releases on his Tzadik label qualify as well.
Much of modern jazz is quasi modern classical at this juncture, that's the jazz that interests me most. I like cross-genre musical adventures.
For jazz in this vein I like Vijay Iyer, Rudresh Mahanthappa, a lot of Miles Davis, Tim Hecker, the list goes on and on.