Unranked...
1. Isaac Asimov
2. Kurt Vonnegut, Jr.
3. Italo Calvino
4. Franz Kafka
5. James Thurber
Beyond this point I'd be stretching it. A few months ago I would have added Connie Willis, but her last few books have been really tedious. I like Dickens but haven't read nearly enough, ditto for Dostoyevsky and Orwell. I've read the Hitchhiker books many times, but I'm not as fond of Adams as I used to be. Salinger and AC Doyle might be contenders, but I haven't read their stuff for over 15 years.