I used to have really horrible nightmares as a child. It's hard to say the worst, but for an example of one of the vivid ones I remember to this day...
When I was about 6 or 8...I had a dream a thief broke into our house. My mom, I assume, was the first to wake...waking up my dad, and they both came to check it out. Where I came in, was to the scene of my mom, lying in my arms, bleeding out from knife wounds. I was covered in her blood, and she was crying. She was speaking to me, trying to reassure and comfort me, even putting her hand to my face...but she died there...in my arms. All the while my dad was fighting for his own life against the same man who was robbing our house and had just killed my mother. I woke up frantic and in tears...my parents were in the door to my room because I was crying and I think I may have screamed or moaned at some point to alert them...I told them all about it and was very clingy to my mother for the next few days. That's an example, but not necessarily the worst.