Okay, so I know technically this isn't just "one person" but I have always found that storylines revolving around teenage peer pressure and cruelty themes to be the most powerful. Episodes like "Wannabe," "Ours Sons and Daughters," and "Clean Up" were always the most upsetting. I think partly because they seemed so real. These were things that permeated everywhere and usually started in the most subtle and unsuspecting way.