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.
"Half my life is over and I have nothing to show for it. Nothing. I am a thumbprint on the window of a skyscraper. I'm a smudge of excrement on a tissue surging out to sea with a million tons of raw sewage."