The 1973 BBC Radio Dramatization of Asimov’s Foundation. It’s about eight hours long and the voice work is quite good. It’s comfortable for me to listen to and come back to, very digestible. One complaint: I’ve yet to find a version that had properly equalized sound levels, so the comfortable listing volume for their speech throughout the work is suddenly jarringly loud when they switch to the machine-clacking “encyclopedia” segments that serve as segues between parts of the story. Other than that, I have no complaints: It’s a fairly faithful adaptation of the original work, and does not suffer from the fatigue and dating many other works do (in my opinion, audio balancing notwithstanding).