MDMA or lsd. Though psilocybin and lsd are physically not harmful to the human body, it can lead to depersonalization, panic attacks, bad thought loops, etc. Though I've personally never had a bad trip on legitimate lsd and I've never done psilocybin, a friend of mine smoked weed while tripping on acid and it caused a pretty bad panic attack. He's tripped on acid without taking anything else and it was all good. He also had a panic attack on shrooms (without taking anything else). MDMA on the other hand, I have never personally met someone who has a had a bad time on MDMA. The big issue is it is easily abused and can actually kill you if you take way too much. Prolonged constant use is also very bad for the brain. Flip side, it's being used in clinical trials to help with PTSD and rolling once every few months is okay. Also feels absolutely amazing.