This is a tough question, but here is my list -- or really a partial list because there are so many more whose books I buy whenever I come across one I haven't already read or bought.
1. Sinclair Lewis (if you haven't read "It Can't Happen Here" you should)
2. L.E. Modesitt (the Recluse series only)
3. Isaac Asimov
4. Harry Turtledove
5. David Weber (especially the Honor Harrington books and his new series that began with "Off Armageddon Reef")
6. George Orwell
7. John Mortimer (his Rumpole of the Bailey books)
I think I'll quit before I start wearing out my fingers
