I agree The Murder of Roger Ackroyd is the best of the books. Leaving Suchet aside (the best of the TV Poirots, easily), my favourite Poirot film is the 1974 Murder on the Orient Express with Albert Finney. Vastly superior to the Branagh version (consider for instance the stunningly eerie and unsettling opening, and the denouement is so much more sinister).