Both Agatha, no? Murder on the Orient Express only ticked me off a little because ALL of the suspects did it - which seemed a little too..... I dunno, cheap? Fun read though!
I couldn't even tell you my favorite Agatha Christie, they're all really good. Though of the top contenders is "Death on the Nile," "The Witness for the Prosecution" (also a killer movie), and "The Miror Crack'd"