There is a decline of quality of audiobooks however, it's often best to find a copy of BBC radio plays that promise a full cast of voice actors, sound fx and music tailored to specific stories and dramatic points. I have found that Brian Sibley and Michael Bakewell have produced some of the best radioplays to date. There are many worth finding, here a few to hunt for...
THE FOOL'S PICKS
1. BBC Orson Welles directed and narrated H.G. Wells' novel War of the Worlds
2. 1981 BBC Radio 4, J.R.R. Tolkien's Lord of the Rings trilogy. A twelve part series is an abridged version starring Ian Holm as Frodo along with many other great voices.
3. 1978 BBC Radio 4 version of Douglas Adams' novel, The Hitchhiker's Guide to the Galaxy
4. 1985 Titus Groan and Gormenghast
5. 1968 The Hobbit by BBC 4
6. Orson Welles 1939 radio adaptation of Agatha Christie's The Murder of Roger Ackroyd
7. The Chronicles of Narnia (10 x 25 min episodes) Among the cast of talent, Tom Wilkinson... recognize the name?
The very best of Agatha Christie audiobooks tend to pale in comparison to their BBC radio play counterparts. I have noticed that John Moffatt was the David Suchet for radio, playing the voice of Poirot more than once. Many of the Christie's novels are read respectably well by Hugh Frasier or David Suchet but nothing beats a well produced play.
Poirot BBC 4 (John Moffatt voiced Poirot 1987 onward with great success)
1985-86 The Mystery on the Blue Train (Maurice Denham)
1986 Hercule Poirot's Christmas (Peter Sallis)
Murder on the Orient Express
Death on the Nile
Murder on the Links
Murder in Mespotomia
Cards on the Table
Appointment with Death
Sad Cypress (This was truly one of their best. Emma Fielding and David McCallister as the two main roles.)
Marple BBC 4 (June Whitfield plays Miss Marple)
At Bertram's Hotel
A Caribbean Mystery
4:50 From Paddington
Other Christie plays
1953 Partners in Crime (Richard Attenborough and Shelia Sims play Tommy and Tuppence), a thirteen part version.
1990 The Sittaford Mystery
1940 And Then There Were None
1993 The Pale Horse (One of my favourites)