As a collector, I have all of his works in hardcover and again in soft cover, (*grin*) I would personally say that his best was "A Time to Kill" if only for the story content. However, for storyTeLLING his best descriptions were done in the novel "A Painted House" and "The Innocent Man". I could give a list from my considered best to worst but perhaps a hub would be better?!
I liked "A Time to Kill"; I thought "The Innocent Man" was frustratingly suffocating and depressing; and "Painted House", good if you like this sort of thing but one of my favorites is still Street Lawyer.