I have no first hand knowledge to rank pilots so wouldn't presume to pick one as the 'greatest' but I do have a vast collection of aviation books, including any I could find on test flying. Any shortlist have to include Eric Brown, Bob Hoover, Alex Henshaw and Neil Armstrong. On reflection, I'm completely awed by anyone who gets to be a test pilot, so you can add Roland Beamont, Neville Duke, Neil Williams and a host of other names to the list...