Zodiac.

Even before this movie, David Fincher had done thrillers.
Se7en, The Game, Panic Room.
Just Se7en practically reinvented the genre.
But yet again a thriller? Could he be moving in circles? No.

He managed to find yet another angle he hadn't done: the "documentary" approach.
By basing this on actual events and on an true investigation, you get another edge on the audience. Like in Fargo (except that here it is actually true, at least the facts are), you tend to give more credit to the story.
The fact that the killer was never caught, that he might still be alive and well, is the most frightening thing. Like Jack The Ripper.
You can invent the most twisted boogie man, it will never ever come as scary as a "real" killer. Especially if he never got caught...

The film basically lays in 2 parts. The 1st presents the facts and the initial investigation by Paul Avery (journalist at the San Francisco Chronicles) on one hand & David Toschi (the cop in charge of the case) on the other.
You get to see the killings and the lack of communication between all parties involved in finding the killer.
Then the 2nd part sees Graysmith, cartoonist at the Post, take over the investigation when he sees that noone will ever finish it otherwise. He had just been interested in the case without being able to contribute to it so far, so he decides to take everything back to 0.

Fincher does masterfully bring that story back to life, thanks to a very faithful recreation of the seventies and a top notch cast: Ruffalo, Downey Jr. & Gyllenhall are all excellent and really carry the film.

And thanks to his excellent direction, he delivers a movie that manages to be anti-spectacular and wordy, but at the same time gripping and always entertaining.
It's quite amazing when you think that you get no action scene and most of the suspence relies on the paranoia of the protagonists (ie there's no direct threat).
But thanks to a great story like only reality can give you and great pacing, the nearly 20 years covered over the rather large runing time (2h30) passes like a charm.

And you get this "it actually happened" chill, when the credits roll...