Anything involving Milhouse or Burns, my two favorites:
Bart - "Milhouse! You were supposed to be my night watchman!"
Milhouse - "I was watchin. First it started to fall over. Then it fell over."
"Take that mom! Take that dad! Send me to a phychiatrist will you! Take that Dr. Sally Waxler!" ::Hits eject button in plane:: BOOM!!!
DING DONG
Burns - "Welcome, come in. Ahh, fresh victims for my army of the undead.
Smithers - "Sir you have to let go of the button."
Burns - Oh, son of a bitch!"
Burns - "As punishment for your desertion it's company policy to give you the plague."
Other great ones:
"Can't talk comin' down." Countless mornings I've said that myself
"Bart, stop pestering Satan."
"Holy flirking schnit!"
"Uh oh... My eyes! The goggles do nothing!"
I could go on and on but I'll stop now. Classic show.