I'd hafta nominate 3-FPM.
At first, I wanted to try it, just for the novelty value of something else you can vape from foil. And for its appetite suppressant property (it's a substituted analogue of the old diet pill Preludin, which alas for this fat cow they no longer prescribe). It scared me a bit, especially the idea that it was so fiendish, but in a nice way; think more big dipper or zombie apocalypse movie than train crash or real live zombie apocalypse.
Then, I had the misfortune to witness someone going through a real, live meltdown on 3-FPM. There was this person -- I won't even say whether she was a man or a woman, let alone who; you either already know or you're better off not knowing -- it was really that bad. I mean, sure, if you don't eat or sleep for a week on the trot, you're going to go a little bit crazy, drugs or no drugs. N.B.: If you ever think that is happening to you, by the way, call a friend and have them sit with you and talk nonsense until you fall asleep. And drink some Dioralyte. Anyway, that particular advice would not have worked in this particular case anyway, for a pretty fundamental reason; and it wasn't for want of Dioralyte. The whole thing was not at all a pleasant sight to witness, from a position that was simultaneously so close and yet so remote. You really cannot buddy someone down from a bad trip unless you are right there in the same room with them and can see and hear everything they can. It's possible to read uncorrected typing errors almost like a kind of body language, but that's not enough.
Anyway ..... It was ..... um ..... It was an attitude-shaping experience. Kind of like how some people (present company included) think rape jokes are not funny because they have been too close to rape, whether it happened to them or to someone close to them, and therefore one of the necessary characteristics for something to be funny -- absurdity -- is absent. Stimulant psychosis was suddenly all too realistic a prospect, knowing that I already had a low tolerance for stimulants anyway. With its reputation for compulsion to redose, and a route of administration that I could sit and use all day long, I could see a recipe for disaster. Not just some abstract concept of a disaster, either; but a real, live human being undergoing a very concrete disaster. I could see precisely the person I least wanted to be.
Shortly afterwards, 3-FPM became unavailable anyway due to the Psychoactive Substances Act becoming law, and I was never going to miss it .