Gumball defeating the evil virus in "The Virus". After it has them cornered after possessing cars and trying to run them down, Gumball challenges it to come and get him by appealing to its ego. After the virus emerges, Gumball simply steps on him. It's even more impressive because Gumball actually won by being smart for once.
In "The Limit" after Richard was tricked into being a man and demands Nicole do as he says, Nicole responds by giving him a Death Glare so deadly, it blows away all the stock on the store shelves.
Richard gets one in "The Hero" for rescuing Gumball and Darwin from the car crusher in the junkyard.
At the end of "The Castle", Nicole forces the people who trashed her house by using a Penance Stare.