Mine would have to be when Kay is describing to Michael her plans on leaving him.
She starts off by simply belittling, or trying to belittle, Michael by basically calling him stupid saying she tricked him. she then continues by telling him it was an abortion, something that she certainly knows is unforgivable. she then "pushes" even more, seemingly, with the pure intent of malice. you can see, so vividly, the morphing of Michael's moods; from remorsefull, to consoling, to disgust, to pure anger. it is brilliance at its best. i can't get enough of it.

and also the scene of the trial when pentangeli has his "change of heart"... when he says "Yeash... sure"... classic

"From now on, nothing goes down unless I'm involved. No blackjack no dope deals, no nothing. A nickel bag gets sold in the park, I want in. You guys got fat while everybody starved on the street. Now it's my turn." (King of New York)