Intro here, No. 5 here, No. 4 here. Would love to hear your most-quoted movie lines below. 

3. “I’m smart! Not like everybody says — like dumb. I’m smart!”
From “The Godfather – Part II” (1974)
Screenplay by Francis Ford Coppola and Mario Puzo

It's a heart-breaking scene, isn't it? At the start of this saga, there are three sons: Sonny, Fredo, Michael. Sonny’s the volatile one, the future godfather. Mike’s the war hero, and, we soon find out, a rather cold bastard. And Fredo is, well, John Cazale. Not particularly attractive, not particularly adept at the family business. While his father’s being gunned down at a fruit stand in Little Italy, he’s doing a Woody Allen bit with his gun; then he slumps, crying, next to his father’s bullet-riddled body. They don’t even bother to shoot him. Think about that for a second. They don’t even bother to shoot him. Ouch. In Vegas, he momentarily takes Moe Green’s side against the family, and in “II” he’s used as a pawn in Hyman Roth’s attempt to kill Michael. Not smart. A liability. 

In this scene, which takes place in the Corleones’ Nevada compound in the middle of winter, Michael is plotting strategy around the U.S. Senate investigation into his affairs, and, needing information, he leaves his office and consults with Fredo in a side room. This is the first scene between the two since Cuba, when Michael found out Fredo betrayed him, and Fredo is, understandably, offering mea culpas and excuses. He sits slumped in his chair, a puppet whose strings have been cut. Eventually, though, he lets loose. We find out how he feels about being stepped over (not good) and how he feels about being errand-boy for the family (ditto). Michael says, in his flat voice, “That’s the way pop wanted it” and Fredo screams, “That’s not the way I wanted it!” By this time his body is racked with almost palsied shaking—compare it with Michael’s glacial cool—and he says the above line, a line that reveals its opposite three times over.

First, it’s hardly Henry James. Grammatically, it’s a pretty dumb way to say you’re smart.

Second, anyone who has to say he’s smart, isn’t. Try to imagine Einstein saying the line. Try to imagine Michael saying it.

Third, hasn't he been paying attention? Hold your friends close and your enemies closer. Never let anyone outside the family know what you’re thinking. By this point, Fredo should be wary of Michael. He should view him as his enemy. And yet he still reveals everything to him. Michael probably would’ve had Fredo killed anyway, but this outburst let him know, as much as anything, how much resentment, and how little self-control, Fredo has. Thus Fredo's final boatride: Hail Mary, full of grace...

I tend to say this line (wrapped, yes, in a bad John Cazale imitation) whenever I realize I’ve just said or done something stupid. When I'm battling against my own stupidity. It’s my third-most-quoted line. You do the math.