How do you pick an example of the best traditional whodunit? Even if you stay with just the cream of the crop--Arthur Conan Doyle, Agatha Christie & Ellery Queeen--the thought of picking out just one example can make your brain explode. There's just too much good stuff to pick from.
But, by golly, I write for a blog which has a bare handful of regular readers at best, so there's no sense in letting my brain explode. I will force myself to choose--more or less at random--Agatha Christie's Belgium detective Hercule Poirot.
All great whodunits have to have appealing main characters. Poirot is vain and egotistical, but he's undeniably brilliant and oddly likable. Retired from the Belgium police force, Poirot travels the world. But wherever he goes, someone is inevitably murdered--the local police are baffled--and Poirot steps in to take a hand.
The novels always have a "Watson" character, someone who assist Poirot and narrates the story. In the 1926 novel The Murder of Roger Ackroyd, the "Watson" is Dr. Sheppard, the local doctor in the small English town where Roger Ackroyd came to an untimely end. It's a traditional and effective conceit, letting the reader get all the same information the detective gets, but obligating us to figure it all out for ourselves, since we are not privy to the detective's private thoughts and theories.
At their best, Christie's best novels feature the best mystery plots ever. She was a master of having the least likely suspect turn out to be the guilty party. In fact, if you pick out the least likely suspect and just assume he or she is guilty--well, you'll still be wrong, because it will turn out to be someone even less likely.
The Murder of Roger Ackroyd is a prime example of this. The revelation of who the killer is will--in all likelihood--make your brain explode. If you've never read it, do so soon. But do not peak at the ending. Allow yourself to properly experience one of the best plot twists that have ever graced the printed page.