Miss Marple, Hercule Poirot, Dame Rutherford
Miss Marple's ability to draw parallels between the characters around the murder with personalities she has met in the past confers a unique but unfair advantage to her over the reader. In a way it is like the "woman's instinct" that Agatha Christie's caricature of herself, Ariadne Oliver, lays her stock on, confidently suspecting each of the characters in turn, of being the perpetrator. Only, Miss Marple's first instinct invariably turn out to be correct.
Contrast this with other Christie mysteries, specially those starring Hercule Poirot. He has access to no information other than what the reader also has. Yet Poirot alone draws the correct interpretations and arrives at the solution to the mystery. That is why I believe Hercule Poirot mysteries are more fair to the reader than Miss Marple's. And that is why I like Poirot better than Miss Marple.
BTW, Christie has dedicated one of her mysteries ("The Mirror Cracked" ?) to Margaret Rutherford, "in admiration". Was this because AC was happy with MR's portrayal of MM? IMO Dame Rutherford's robust personality and her adventurous portrayals could not have been more in contrast with Jane Marple's slim, gentle, and "fly on the wall" personality portrayed in Christie's books. Rutherford's performances as Miss Marple were more like caricatures, and not convincing at all, as far as I am concerned.
My first encounter was as a teen, visiting a friend. I was an awful snob, and when she offered me the book I slapped it across the room. (She was a niece of a friend of my mother, and I spent several visits with her - looking back, I can't understand why they had me back after the first visit). But later I read it. I didn't like AC at first - the writing seemed shallow to me, because of the simplicity of the language. Only when I grew up did I learn to appreciate her.
She looked like a bag lady who got her clothes at the Salvation Army. Everyone knows how important appearance was to Miss Marple. Despite her age she was very careful about how she did her hair and the clothing that she wore.
These productions were a travesty to the literary legend of Miss Marple.