Any type of NBOMe. I tend to think any psychedelic can be valuable...NBOMes are the exception to this. Not only do they completely lack any depth or mysticality, but the headspace they have is just...fucking weird. Just an awkward trip, really- not "bad", but certainly not good (I must admit I did have a few good times on them- but that's all they were). Very megalomaniacal and demented. Basically only useful as a party drug, and that's the exact opposite of what a psychedelic should be. And on top of all that, they're among the most dangerous psychedelics and cause the longest tolerance. Fuck that. Only psychedelic I'll ever flush down the toilet!
Diphenhydramine/Dimenhydrinate- did a handful of times when I was younger & dumber (15-16), how anyone could possibly like this shit is beyond me. Pure discomfort, confusion, and weird in a way that shouldn't exist. Deliriants are basically just a trip to hell.
Cocaine- just utterly fucking worthless. A cup of coffee does more for me than any amount of cocaine. How the fuck people spend so much on such a short lived, pointless high is beyond me- especially when psychedelics, dissciatives, marijuana, amphetamines, etc, exist. It's more fun putting coke on my gums so they get numb then it is snorting it.
MDMA-certainly more enjoyable than the others on this list, but not worth ingesting to me. Not gonna throw my serotonin levels out of whack for a buzz I'm not interested in (especially not considering I'm basically a weekly user of psychedelics). Pretty shallow drug if you ask me; admittedly, that's probably got more to do with my personality/mind than anything else...but it's still true for me, and I know I'm not the only one. Will say though that MDA is pretty cool, MDA I want to do a few times more (combo'd with LSD, DMT, 4-sub tryptamines- all separately, though perhaps DMT alongside the others- don't need to do it on it's own though).
One I'm on the fence about is Salvia. I've never broken through on it, sub-breakthrough doses tend to have extremely uncomfortable body highs, or they're just pointless/underwhelming...but I kinda wanna see what a Salvia breakthrough is like...yet at the same time, DMT can do that better, and I've loved all my DMT trips, would easily consider it one of my favorite drugs (and I haven't even managed to breakthrough- have gotten a lot of feelings/vibes from it described in breakthroughs though, and still get unique & POWERFUL trips from it...guess I'll have a full breakthrough when the time is right), so might not ever bother with Salvia again.
Oh yeah also cigarettes...don't even know how/why I used to enjoy smoking them anymore!