Bart and Milhouse get drunk at a Beer festival and are stumbling around in a drunken stupor. Milhouse walks up to Bart and puts his arm around him (imagine a drunken Milhouse saying):
"This guy here. This is THE guy!"
Gets me every time!
This may already be quoted, but I love:
When Marge is scolding Bart for something and Homer joins in to complain about Lisa:
"Look. Lisa made this perpetual motion machine. It keeps going faster and faster! (shouting) Lisa, in this house we obey the laws of thermo-dynamics!"