He was an extremely prolific composer, a unique quality in the twentieth-century. As a result, however, I've had trouble finding a work that I really identify with. What I've heard is all very fun to listen to, but I haven't heard anything just yet that really sticks. That said, both his fourth symphony and fourth chamber symphony were great works. I'm not sure which I prefer. The fourth symphony has frequent use of percussion, which I love in orchestral works, and doodles on some marching tunes, but his fourth chamber symphony sounds much more vibrant, inspired, and lively. The two were composed about twenty years apart.
Definitely worth the listen if you're interested.