In general, Miss Marple's sheer capability for detective work is in of itself Awesome.
The Books
In The Murder at the Vicarage, Miss Marple's excellent detective skills are Played for Laughs when she abruptly enters the room of a murder attempt. How on earth did she know to be there? When an officer called her number by accident she realized there might be something wrong, and headed over to the suspect with the closest phone number to hers.
In The Moving Finger, Miss Marple only appears for the last third of the book, but figures out who the real culprit is almost immediately.
In A Murder is Announced, she manages to figure out the identity of the killer from extremely small discrepancies.
Special mention must go to side character Mitzi. She takes advantage of her reputation as a chronic liar, and flat out tells the murderer that she knows who she is in front of a group of people, banking on the knowledge that she'll either try to kill her or blackmail her. Both of which Miss Marple has been positioned to witness.
When the killer decides to murder Mitzi, Miss Marple imitates her dead friend to distract her.
A Pocket Full of Rye: When a murderer kills her former maid, Miss Marple basically goes on a Roaring Rampage of Revenge the best way she knows how: finding the killer and having them executed.
4.50 from Paddington: The way Miss Marple has her witness identify the killer: she puts herself in the exact same "being strangled" position as the victim, and has the killer check her throat.
Sleeping Murder: Miss Marple prevents a murder by blasting the killer with a hose.
Nemesis, where Miss Marple (figuratively) declares herself the living embodiment of the Greek goddess of revenge to the murderer.
In Murder Ahoy; a killer with at least 3 victims under his belt knocks out the only other sailor on the ship and prepares to attack Miss Marple himself. Miss Marple promptly grabs a sword and fends him off long enough for her assistant to arrive.
Mr Stringer's Big Damn Heroes moment at the end of the fight certainly qualifies.
Murder Most Foul is adapted from a Poirot mystery, proving that yes. Miss Marple is on the same level of genius as him.
Ditto with Murder at the Gallop. Bonus points for being a more active investigator than Poirot this time around, risking her life and snooping around to find the killer.
