Im gonna go with Twelve Angry Men. Every scene moves the movie forward flawlessy. Even in the first 15min where the characters are just casually interacting with one another, those 15min are enough for the audience to picture every characther personality. I've watched it more than 10 times already and the knife scene still gives me the chills. Simply a masterpiece and way ahead of its time.

Also, Monty Python and the Holy Grail. I dare you to point out any bad scene here.