Well, I have little in terms of "worst" nightmares go as I have more or less forgotten about them or have grown out of them. But if I had to name one, then I suppose poltergeist-esque nightmares/activities still scare me shitless sometimes.
Though, perhaps one of my weirdest ones in my entire lifetime, was a dream of me running around my elementary school building while evading two "girls" whom seemed to have the power to teleport around freely as they saw fit.
I don't know how, I don't know why, but I ran out of fear. I only remember running out of fear, while hearing chatter and playful tones coming out of their mouths as they talked in the distance, or close to me. Running away, hiding, fleeing in bursts. It was one of my weirdest yet somehow most memorable and enjoyable ones now that I think about it.