I had a pretty intense trip on acid that took me about two weeks to get over once. But, it probably wasn't the worst for me.
The scariest was probably dramamine. Me and two friends took 24 each and my one buddy was kinda skinny and short and it was his first time trying it, so it hit him significantly more intense then me and the other person. He became completely delirious with absolutely no ability to competently communicate at all, made rather grotesque looking spasm motions, and he pissed himself. Meanwhile, I was seeing dead cats and hearing voices and shit. He then just walked outside and sat in the middle of the street babbling random words and staring off into nothing. I went outside to get him, but my other friend was tripping so hard, she locked us out and hid in her dad's room for hours. I was still somewhat aware of reality, but was buggin' pretty hard as well. Plus, seeing my friend go batshit-insane scared me and increased my heart rate. Then, I realized I hadn't seen him in hours.
I saw a dark silhouette crouching as if holding a gun and thought I was really losing it, but eventually realized that there were several cops rollin' up, swat-style, on a vehicle. My buddy, in a state of delirium, had climbed into someone's car and they called the cops. They couldn't understand him and took him to the hospital. He was fine the next day though.
Dramamine is fucking nuts. Intensely realistic and vivid hallucinations, delirium, hearing voices, loss of control over motor skills and speaking. I had done it a few times before that and had interesting trips. Not really "fun", but intriguing and rather fascinating. However, that time made me realize that the shit is not to be done recklessly.
I also once took 5 adderall and sincerely thought my last moments of life were occuring. Heart palpitations and an extremely brutal migraine. Never fucked with it again.