Well, other than myself
, I'd have to vote for Michael Crichton. The breadth of subjects that he tackled is what impresses me the most; who else can write on technology, history, medicine, and contemporary politics without losing his readers. As far as classical authors go, my favorite is Charles Dickens because of his amazingly astute descriptions of both personalities and personality disorders, not to mention of many physical disorders that medicine hadn't even described yet. For instance, "Pickwickian" is actually a medical term that is used to describe obese and air hungered patient's-- a term derived from Dickens' The Pickwick Papers.
Jules Verne is another of the greats that I'm a big fan of. In many ways, Verne was to the 19th century as Crichton was to the 20th, with regard to technological writings anyhow. If you've never written a book before, it's hard to appreciate how incredibly difficult it is to create an entire world like Tolkien did; at least I know that I didn't fully appreciate it until I started really writing.