My favorite scenes in Pretty in Pink are, pretty predictably, all the ones involving Ducky. Obviously, the disc shop scene is a monument, as is the one when he talks to the bouncer. But what strikes me in both cases is how little dramatic justification these two scenes have. They're there just for the pleasure of spending time with a character who ends up being a lot more than just the foil to the girl (which James Spader ends up being for the boy, although his charisma saves him). Duckie is the one stray element in the film, infinitely more physical than Molly Ringwald or Andrew McCarthy, and though he's probably, with Harry Dean Stanton, the one who suffers most in the film, he's the one most capable of inhabiting his body with sheer joy.
Likewise, Ferris Bueller's Day Off is one long string of scenes that are just for the pleasure. Which is why the least convincing bits (though they are still fun) are the ones involving Jeffrey Jones once Sloane has been picked up from school. They're there to provide some sort of narrative stake, which goes against the grain of the rest of the film: that day off is one big digression.
Also, what is it about museums and perfect scenes? Two great films find their souls in them.