We Haven't Had A Good Whodunnit Since 2001
What We Used To Have:
Clue, Rear Window, Ten Little Indians, Gosford Park ... fuck it, let's count Murder At 1600.
What We Have Now:
That new Murder On The Orient Express ... provided it doesn't blow a turd.
I should be clear about what I mean by "whodunnit," because the murder mystery genre can encompass a lot of things. At its purest, it takes place in a small location, preferably with characters who are unable to leave. There should be at least one mustache. Also, it's important to have an interesting cast of suspects, and enough evidence for the audience to at least feel like they can solve the mystery in question on their own. The last iteration of this formula that I recall has to be 2003's Identity, and the last non-garbage one is likely 2001's Gosford Park, which featured actors like Michael Gambon, Clive Owen, Maggie Smith, and Charles Dance in a Downton Abbey-like mansion where a fancy-pants murder has taken place.
Don't be mistaken, since this genre has dwindled, there are a lot of detective thrillers you might confuse for the same thing. Movies and TV shows like Se7en, The Girl With The Dragon Tattoo, True Detective, and The Killing are all wonderful, but often conclude with the killer being some background Waldo from a single scene we long forgot about. Other times, we get movies like Shutter Island, which start promising but end up betraying their genre for some crazy Fight Club ending.
See, the classic whodunnit, while having some twists and turns, doesn't need an outlandish narrative u-turn to be enjoyable. The appeal lies in the characters and tension of the environment. But as filmmakers kept making these movies, there was a weird pressure to force various third-act gimmicks to keep the premise fresh. A good example has to be Scream, which I would argue is a horror version of a whodunnit (complete with a mustache from Dewey). As they kept making sequels, the writers kept trying to make the final reveals more and more elaborate and twisty, making the killer impossible to determine ahead of time due to a lack of necessary information being given to the audience.