Not truly an audiobook, but the BBC has a performance of The Lord of the Rings that's really good. It even has Ian Holm (Bilbo from the movies) in it. But he was younger then, so he plays Frodo.
I have yet to listen to Andy Serkis do LoTR, Rob Inglis does a great narration, but the fan-made, unauthorized, copyright infringing masterpiece by Phil Dragash is the best imho.