Sleeping Murder is my favorite, but I think partially because it was the first one I read as a kid and kicked off my Agatha Christie reading (I was moving on from the Three Investigators and the Hardy Boys).
I also seem remember liking Murder at the Vicarage, Murder in Mesopotamia,By the Pricking of My Thumbs, and What Mrs. McGuilicady Saw. I also like d Nor M, but suspect as an adult it might not be as good of a read. PBS just did a series of Tommy and Tuppence, but I think the casting was off for the leads.
The one book I did not like as a kid was Postern of Fate. I remember thinking Passenger of Frankfurt was a little off, but Postern of Fate was the one I had a hard time actually finishing. You could tell she was less in tune with the modern world and her best books were probably pre-1950. Sleeping Murder and Curtain were the last published, but she wrote them years before to be the last Porioit and Marple books published.
I had not read her in years, and recently picked a couple up at a garage sale. Curtain - which was pretty good, but Poriot bugs me at times and his friend is just a little too dense. Nemesis, which I thought was one of the weaker ones. The world she grew up and and knew how to write about (basically pre WWII fading empire Britain) did not mesh well with more contemporary settings.
While not as big of a Poriot fan, Death on the Nile is probably one of the better ones. Then There were None is also one of her classics, but might suffer from being done so many times, not just in movies of the books, but as a plot from dozens of TV shows from the 60's, 70's, and 80's - from Remington Steele to Wild Wild West.
I have not read Murder on the Orient Express, largely because I saw the original movie and know he ending, but I think that is considered one of her classics as well.
Sleeping Murder is my favorite, but objectively speaking, Death on the Nile and Then There were None were probably better books.
Postern of Fate, Nemesis, and probably Passenger to Frankfurt would be the ones I would not recommend.
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