In my opinion Henrietta Savernake ("The Hollow") is kwa far the most likeable and interesting suspect in a HP novel. It has been stated (by Robert Barnard) that Agatha Christie disliked professional women. If he's right, Henrietta must be the exception. A wonderful woman!! The most interesting murderer is very difficult to point out. But Jane Wilkinson ("Lord Edgware dies") and Dr. Sheppard ("Rger Ackroyd") are serious candidates
