I've read them all (bar Poirot's last case) over the years and every time I can't think what to read I go back and read one of them again. Love the descriptions of the characters, the settings, the clothes, all of it!
First one I read in my teens was The Murder of Rodger Ackroyd and I still think it's one of the best whodunnits I've read.
Anyone else have a favourite AC book?