The most famous one from that seminal 1972 film is "I'm gonna make him an offer he can't refuse."

But I like this one from Michael Corleone to his brother-in-law Carlo when trying to elicit a confession that Carlo set up Sonny Corleone (Michael's older brother) to be killed:

"... Don't tell me you're innocent. Because it insults my intelligence. It makes me very angry."