That would be "Harriet Klausner", who has written (so Amazon.com claims) 15,358 book reviews for Amazon.com since 22 November 1999. That's an average of 5.2 book reviews per day, by the way, which makes Harriet Klausner either one of the most prolific readers on the planet or else one of the most thinly disguised marketing strategies ever designed by the pulp-fiction industry. And she's not just the No. 1 Lady Amazon Book Reviewer, she's the No. 1 Reviewer overall, her next closest competitor being "Lawrance M. Bernabo", with 6666 reviews written since 30 August 2000 (a mere 2.5 book reviews per day).
I'm shocked--shocked!--to find that some of the writers of comments to "Harriet Klausner's" reviews suspect that she is either not a real person, or not a single person, or just a bored librarian banally copying blurbs off the backs of third-rate detective novels as they come in to her library (the vast majority of her reviews are of detective novels, and her average rating is five stars). Imagine Amazon.com doing a thing like that, just to help publishers sell books. It's downright...commercial!
Full disclosure: "Harriet Klausner" did not actually review The No. 1 Ladies' Detective Agency by Alexander McCall Smith. Too bad: if she had, I'm sure she would have given it a well-deserved five stars.
Oh, and I am Reviewer number 45,693.