Hmm, tough call. I agree, to introduce Miss Marple, and being that St. Mary Mead is where all her insight comes from, I'd say "Murder at the Vicarage" would be a good start for her. It would also establish characters who could return in later games. The Christie Games website had an opinion poll after And Then There Were None was released and I voted for Orient Express. But they also mention "Death Comes As The End." Though not one of her better mysteries, it's a classic tale, set in ancient Egypt with a wealthy family slowly being murdered one by one, as if under some ancient curse. That would make an excellent adventure game, IMHO.
"Do not allow evil into your heart, it will make a home there." - Hercule Poirot from "Death on the Nile."