The following are listed in Robert Barnard's book "A Talent to Deceive: An Appreciation of Agatha Christie" as being in his opinion among those on the top rung of Agatha Christie's works (of course your opinion may differ). The short comments on each novel below are taken directly from the appendix of Barnard's book. (If you don't want to listen to all of Christie's 66 novels, then at least try this dozen!)
The ABC Murders (1936) narrated by Hugh Fraser, 6 hours 1 min.
"A classic, still fresh story, beautifully worked out. It differs from the usual pattern in that we seem to be involved in a chase: the series of murders appears to be the work of a maniac. In fact the solution reasserts the classic pattern of a closed circle of suspects, with a logical, well-motivated murder plan. The English detective story cannot embrace the irrational, it seems. A total success – but thank God she didn't try taking it through to Z."
Cards On The Table (1936) narrated by Hugh Fraser, 6 hours.
"On the very top rung. Special opportunities for bridge enthusiasts, but others can play. Superb tight construction and excellent clueing. Will be read as long as hard-faced ladies gather for cards."
Death On The Nile (1937) narrated by David Suchet, 8 hours 42 mins.
"One of the top ten, in spite of an overcomplex solution. The familiar marital triangle, set on a Nile steamer. Comparatively little local colour, but some good grotesques among the passengers – of which the film took advantage. Spies and agitators are beginning to invade the pure Christie detective story at this period, as the slide towards war begins."
Murder Is Easy (1939) narrated by Hugh Fraser, 6 hours 5 mins.
"Archetypal Mayhem Parva story, with all the best ingredients: Cranford-style village with 'about six women to every man'; doctors, lawyers, retired colonels and antique dealers; suspicions of black magic; and, as optional extra ingredient, a memorably awful press lord. And of course a generous allowance of sharp old spinsters. Shorter than most on detection, perhaps because the detection is, until the end, basically amateur. One of the classics."
And Then There Were None (1939) narrated by Hugh Fraser, 5 hours 37 mins.
"Unusually suspenseful and menacing detective-story-cum-thriller. The closed island setting with the succession of deaths is here taken to its logical conclusion, and the dangers of ludicrousness and sheer reader-disbelief are skillfully avoided. Probably the best-known Christie, and justifiably among the most popular."
Sad Cypress (1940) narrated by David Suchet, 6 hours 2 mins.
"A variation on the usual triangle theme and the only time Christie uses the lovely-woman-in-the-dock-accused-of-murder ploy. Elegiac, more emotionally involving than is usual in Christie, but the ingenuity and superb clueing put it among the very best of the classic titles. Her knowledge of poison is well to the fore, but the amateur will also benefit from a knowledge of horticulture and a skill in close reading."
Five Little Pigs (1942) narrated by Hugh Fraser, 6 hours 42 mins.
"The-murder-in-the-past plot on its first and best appearance – accept no later substitutes. Presentation more intricate than usual, characterization more subtle. All in all, it is a beautifully tailored book, rich and satisfying. The present writer would be willing to chance his arm and say that this is the best Christie of all."
Towards Zero (1944) narrated by Hugh Fraser, 6 hours 15 mins.
"Superb: intricately plotted and unusual. The murder comes later, and the real climax of the murderer's plot only at the end. The ingenuity excuses a degree of far-fetchedness. Highly effective story of the child and the bow-and-arrow (part II, chapter 6) and good characterization of the playboy-sportsman central character – very much of that era when one was expected to behave like a gentleman at Wimbledon."
The Hollow (1946) narrated by Hugh Fraser, 7 hours 12 mins.
"Notable specimen, with more complex characterization than usual, and occasionally rising to wit (especially on the subject of cooking). Illustrates vividly one dilemma of the detective writer: if you establish characters of some psychological complexity, how do you prevent the routine detection stuff coming as an anticlimax? .... Definitely among the top ten, in spite of the falling-off in the second half."
A Murder Is Announced (1950) narrated by Rosemary Leach, 8 hours 44 mins.
"Superb reworking of the standard Christie setting and procedures, marred only by an excess of homicide at the end. The book is distantly related to The Companion, in The Thirteen Problems."
Ordeal By Innocence (1958) narrated by Hugh Fraser, 7 hours 6 mins.
"One of the best of the 'fifties Christies, and one of her own favourites (though she named different titles at different times). The Five Little Pigs pattern of murder-in-the-past, the convicted murderer having died in prison, innocent. Short on detection, but fairly dense in social observation. Understanding in treatment of adopted children, but not altogether tactful on the colour question: 'Tina's always the dark horse…Perhaps it's the half of her that isn't white."
Endless Night (1967) narrated by Hugh Fraser, 6 hours 14 mins.
"The best of the late Christies, the plot a combination of patterns used in Ackroyd and Nile (note similarities in treatment of heiress/heroine's American lawyers in Nile and here, suggesting she had been rereading). The murder occurs very late, and thus the central section seems desultory, even novelettish (poor little rich girl, gypsy's curse, etc.). But all is justified by the conclusion. A splendid late flowering."
All are unabridged mp3s at 48 kbps / 44 kHz Mono CBR, Fraunhofer MP3.