My first Agatha Christie book was "And then there were None." I enjoyed it tremendously...the sense of mystery in her books are infectious. I've only read:
And then there were None
Ordeal by Innocence
ABC Murders
The Mystery of the Blue Train
A Murder is Announced
Appointment with Death
The Mysterious Affair at Styles
The Clocks
Death in Clouds
Seven Dials Mystery
Sad Cypress
...and some more,of which I've forgotten about I think. I've read the theater play versions for "The Mousetrap", "The Hollow", "Appointment with Death" and "And then there were None" (different ending).
I prefer Hercule Poirot because I've read most of his mysteries and only one of Miss Marple's. I need to read some more of her someday. ^_^