Personally I have 3 favs and can't choose between them.

1] The scene where Pesci gets someone's skull in the middle of a metal tool (G-shaped thing used mainly by mechanics and carpenters)...That was THE first time I've seen such a scene.

2] When Pesci meets with the banker in Deniro's house and gives him the speach about cracking his skull open twice.

3] When Stone asks Pesci to whack Deniro and he barks at her: "...I've known the guy for 35 years and now I'm gonna whack him for YOU???" I was always confused at Pesci's real concern here: Was he concerned about killing his friend? Or was he scared of the bosses back home.