The Misfit does not have a phrase in this story that he repeats again and again. Therefore to answer your question it is important to identify what the most important thing is that the Misfit says as he talks to the grandmother. For me, it is the following saying:
"No pleasure but meanness," he said and his voice had become almost a snarl.
This is significant because it describes to us the way in which the Misfit lives his life. He finds no pleasure at all but in commiting evil acts, and this is what defines him as a "Misfit." Yet what makes this quote from the Misfit so important is the way that, having just said this, he then goes on to contradict himself shortly after killing the grandmother. When Bobby Lee says "Some fun!" to describe the killing of the family, he is told to shut up, as "It's no real pleasure in life." This is significant because it reveals that, having declared his position as finding only pleasure in evil, the conversation with the grandmother has made him realise that there is in fact no pleasure in such deeds. Even a character such as the Misfit demonstrates that he can be an unlikely recipient of grace.