Most anxiolytic psychedelic is, by a massive margin, nitrous oxide.
There really aren't even any close competitors, given its 4 pronged action at:
GABA-A: binding @ benzodiazepine binding site
NMDAR: reduced glutamatamic arousal
MOR: Endorphin release
Mesolimbic D2R: DA release
And for those who say that nitrous oxide isn't a real psychedelic: you simply haven't had enough of it. Like any drug, levels of N2O build in the blood, and it takes a sufficient blood level for the true psychedelic effects to come out. At low doses (a few cartridges) it mainly causes perceptual changes (the auditory effects being most talked about). Only after inhaling a constant stream of cartridges (best inhaled directly, not with a balloon) does the N20 headspace emerge. I'd say it takes a minimum of 10 cartridges in short succession to begin to enter the headspace of NOS. It takes a good 30+ to actually start truly tripping. Like with DXM, the nitrous oxide psychedelic experience manifests in different "plateaus".
Of course, this isn't especially safe. Nitrous oxide inhaled in that quality can cause significant depletion of blood oxygen, and of course, regular use causes B12 depletion. I used it daily at an average of 50+ cartridges a day for around a year and a half. I actually began using it to help with benzo withdrawal symptoms, since its rapid action on the GABA-A BZD binding site leads to brief but instant relief. And as an opioid addict, the endorphin release was welcomed. But the psychedelic effects, which are exclusively NMDAR mediated, were kind of a side-effect that actually exacerbate benzo withdrawal symptoms.
I've had experiences where I spoke to god, traveled the universe etc, but the big limitation is that due to GABA-A activation, it is inherently an amnestic drug, and it is very difficult to remember the totality of the psychedelic experience.
Anyway, nitrous oxide is, by wide margin, the most anxiolytic psychedelic. Second might be IV ketamine (which is odd, because IM ketamine can be anxiogenic).
As far as serotonergics, 6-APB takes the cake. But not without its own toll (the risk of 5-HT2b mediated heart damage).
I once plugged 600mg of 6-APB in the hopes of a profound experience. It wasnt what I hoped. The next day when I dragged myself into my office a colleague of mine said, "woah, you look like shit". I felt like i had been hit by a train, was completely wiped, and felt worrisomely weak (physically).
To this day, I wonder what damage that massive dose of 6-APB might have done to my heart valves. I suppose if it had it might have manifested by now. Or will I randomly have a heart attack at 45?