i dont know about "worst" nightmare, ive always been beyond intrigued by the dream world, so even my scary ones have some allure to them that i cant help but find so much interest and wonder, even if i am deeply unnerved.
wouldnt call this a nightmare but when i was younger (17, 18?), for about a month, i would randomly wake from my sleep around 2-3am. my eyes would automatically open and i would lie still as if i had not stopped sleeping. i would hear a ringing in my ears, growing increasingly louder, until it reaches "max volume" (disturbingly loud), and then it would suddenly cut out and go to total silence. this would always freak me out and leave me feeling unsettled until i could fall back asleep, which would be rough.
i have died in dreams a few times. the first ever, was a very long and vivid dream which i can recall with great accuracy. it had a sort of spiritual undertone and was very relevant to something going on in my life at the time, it would be another post in itself. i guess i could share it here if anyone wanted- there were a few other dreams in which i died afterwards, but with less impact, and in general, more intrigue. the best/weirdest thing is, in all these dreams which i died, the dream would keep going. you would expect that you jolt awake at the part where you die, but mine would continue to play out. one dream, i kept being put on this dark highway and had to drive off and avoid some murderous hitchhiker that somehow kept getting into my car and killing me. i would 'die' and 'respawn' back down the highway, driving again. in this dream i retained the knowledge that i had just 'died' and was fully aware of what has happening (in my dream).
another death dream, i was also driving, or my friend was, and we had some other friends in the car. we were on a bridge that abruptly ended and as we skidded to a stop, we tipped over the edge of the bridge and i 'died' when we hit the water. the dream kept going and i could see my mother crying and saying all this stuff, wishing i were here, and i was trying to let her know i wasnt even dead, that it would be ok, but it was one of those ghostly moments where the other person doesnt see or hear you. it was sad.
by far, the most frightening thing that happened, and i guess we can classify this one as a nightmare or night terror, i dreamt that i was in my bed, exactly as i had been sleeping. and i was dreaming my entire room, down to every last detail. i may as well have been awake, thats how "clear" the image was, aside from the fact that everything was very dark (imagine lowering the brightness of a video). anyway, in my dream i was in bed laying down, looking at my window. i was getting a very bad vibe from my window, and there was a lot of visual distortion/haze, almost like tv static/grain. through the entirety of this "dream" i was hearing the most nightmarish sound imaginable, i cannot remember how it sounded, or even begin to describe it. just imagine some eerie, ominous, low droning, nightmare noise. a noise alone that was enough to make me want to shit myself. then the window became this "black hole"- the darkness and "grain" aspects were spiraling in, like a void. a very dark demon hand or claw came from the center, toward me, and when it grabbed me, the nightmarish noise sort of did a "crescendo" in sync and REALLY scared the fucking life from me. i was jolted awake at that moment and sat in bed feeling all kinds of disturbed.