Please pay attention, this is not about which one you liked more or which film was more emotionally involving for you. It's only about which one you've found more challenging in terms of provoking thoughts.

PS: I probably should've only add Memento, Inception and The Prestige on the poll, but who cares, let's put something for trolls.

Memento.

The Dog (2009)

Memento definitely has the smartest ideas, TDK comes in a close second.

Memento. The more time I spent analysing it for an essay, the more I realized I could never go deep enough.

While not my favorite Nolan film, Memento is definitely the smartest.

I would vote The Prestige if it wasn't an adapted film.

Memento it is then.

what do you mean by "provoking thoughts?" I agree Memento is the most challenging film to watch, and that it requires the audiences' attention moreso than Nolan's other films, but I don't necessary think it's thought-provoking. I've never thought about it this way, but I'd probably have to say The Dark Knight is his most "thought-provoking" for me because it deals with more important social/political themes than his other films. Torture/Interrogation-technices; Eavesdropping/Analogy to PATRIOT ACT; sacrificing one's image for hope/security.

Batman Begins. Smartest in that it did justice to the Batman mythos and began the single greatest comic book trilogy we'll probably ever see.