I couldn't agree more re: Mark F. Smith, I've recently listened to a few classics read by Mark F. Smith of South Carolina.
Adventures of Tom Sawyer / Huck Fin , Robinson Crusoe, Treasure Island - White Fang, Call of the Wild and others. He does a brilliant job of the voices - Especially with Long John Silver in Treasure Island (Brilliant).
I found Ralph SNelson a bit more difficult - I think, for me, his voice is a bit too 'thick' for the want of a better word. I recently listened through "The Land that Time Forgot" and really struggled at times. Admittedly I tend to listen in noisy environments such as when driving home from work or out walking or in the gym. The pitch of his voice seems to drop too low and the pronunciation is not always crisp enough for me to deduce what is being said, without upping the volume and rewinding a bit. It might be that his voice is better suited to dedicated listening, however, when I have that kind of quiet I'm usually reading the normal way ;-)
Regards,
Greg