"Helga on the Couch", Hey Arnold.
It's the one focusing entirely on Helga as she's forced to go to counseling after getting caught punching Brainy.
Hey Arnold always tackled great issues, but an episode showing that Helga's anger stems from her neglect was so brutally real. Her parents fawn over her perfect sister and basically forget that she exists, to the point where as a preschooler, she walks by herself in the city in the pouring rain to get to school. She lashes out in anger because it's the only way she's learned how to cope, and her obsession with Arnold comes from the fact that he was the only person to show her any kindness.
It's an excellent episode, but definitely touches on some very real themes.