When Cell tells Trunks why he's hosting the Cell Games.
Perfect Cell: You might say that my only goal now is to enjoy myself, and of course I enjoy nothing more than the looks of perfect terror my power inspires on people's faces.
(cut to Trunks' terrified face)
Perfect Cell: That's it. That's the look!
After Goku arrives on Earth in a Yardratian space pod.
Goku: Vegeta always said there were anymore saiyans left.
Vegeta: Fool! What part of "our planet exploded" don't you understand!
Goku's reaction when Trunks tells him who is his mother is priceless. He actually falls down on the floor when Trunks spills the beans.
At the end of the Cell Games saga. Krillin is attempting to talk to Android 18, and already making a mess of it when Gohan interferes.
The build up to Hercule's battle with Cell. From Hercule karate chopping fourteen tiles of fifteen tiles to Cell's completely deadpan "You can not be serious", culminating with Hercule charging towards Cell, ready to attack. Cell stays where he is and pimpslaps Hercule out of the arena with a single attack.
The Z-Fighter's reaction to Hercule breaking fourteen tiles: A mass WTF uhhhhhhhhh...
Chi-Chi's reaction to seeing Goku and Gohan in Super Saiyan form:
Chi-Chi: NOOOOOOOOOOOOO! MY SON LOOKS LIKE THE FRONTMAN FOR A NEW WAVE PUNK BAAAAAND!