I'm going with my favorite scene where Joker's with the Gambol and the gangsters croonies are on the ground on their knee's while the Joker has the gangster with a knife in his mouth. Then the dialogue is classic!

You want to know how I got these scars?

My father was a drinker and a fiend and one night he goes off crazier then usual.

Mommy gets the kitchen knife to defend herself. He doesn't like that.

NOT ONE BIT!

So, me watching, he takes the knife to her, laughing while he does it, he turns to me and he says,

Why so serious!?

He comes at me with the knife

Why so serious?!!

Stick's the blade in my mouth, "lets put a smile on that face" annnnnnd...

Why so serious?

Then, after he kills Gambol he turns to his croonies and says there is one opening and they have try outs.

Although, the disappearing pencil is awesome!

Lopaka