Timmy when he sacrificed himself to the Darkness to keep his friends, family and even enemies safe. Although, Trixie's tears over his sacrifice were heartwarming, the looks on Chester's and A.J.'s faces were even more touching. They didn't need to say anything nor did they need to cry; the shattered look on the boy's faces pretty much said it all. In fact, their expressions may approach being something of a Tear Jerker.
Even if he ended up making things worse, the fact that Cosmo got ticked off enough about the threat to his wife, his son, and his godchild (and himself) that he turned into Cos-zilla and stepped on the robot.