The Hyperion books by Dan Simmons. Hyperion has full cast, the rest doesn't, but still very high quality narration.

Honourable mentions:

• Alastair Reynolds and John Lee collaborations. Lee has the perfect voice and tone for Reynolds' precise (and somewhat aloof) language.
• Neil Gaiman books. Gaiman does his own narration, and I can't imagine his books without his voice.

Dishonourable mention:

• John Scalzi, read by Will Wright. Wright reads as if every line in Scalzi's works was hillarious. Scalzi is a very funny writer, but his books have somber set pieces, and Wright ruins the immersion there.