Hm, one drug for the rest of my life? I could maybe manage one per class of substances, but not just the one.
For opiates it has to be the mixed bunch of poppy alkaloid acyl esters that results from acylating a refined, cleaned up and recrystallized harvest of opium w/propionyl chloride. Fucking potent as the stench that emanates from a toilet in Gehenna after being filled with feces and vomit and left to moulder away in the heat of hells flames. Strong, fast acting when used IV (a little slower than H, but quite a bit faster than morphine sulfate) and it lasts a LONG time. That propionylated full spectrum opium alkaloid isolate has to have the longest legs of any opiate/opioid I've ever tried; to the extent that even after a bit longer than 10years ago I've been slowly (not slowly enough!, :/) building up tolerance to the big, juicy multiple-choice buffet of pain meds my doc has had me on, and as such, many opiates don't last long enough at all.
The opium isolate though...damn. 10-15mg or so was enough to hit me like a battering ram, and to keep going all day long, in fact it held me out of w/d until the day AFTER taking it in the morning, just a tiny point a little larger than the size of a match head, shot that up and got pinned to my chair with the wind knocked from my sails for a moment the rush was so intense.
Dissociatives? 3-hydroxy-PCP without a doubt, or if not available then methoxetamine.
Stimulants? I.V ethylphenidate
GABAAergic sedatives? allosteric modulators, then chlormethiazole, orthosteric agonists, I can only really comment on the one I have tried, there do not seem to be very many of them to choose from, but muscimol. Fly agaric is a strong medicine indeed, and its recreational/anxiolytic/hypnotic/dissociative-y effects are very enjoyable in their own right.
Cannabinoids, excluding weed itself? HU-210, a very, very potent full agonist at CB1Rs, active in the submilligram area, and it lasts a fucking loooooong time, all day long or into the following day without difficulty, and I found it unusually intensely psychedelic and very visual for a cannabinoid. Or perhaps CP-55,940, which was all around smooth easygoing fun stuff.
Serotonergic psychedelics? tryptamine-wise then vaped DMT without a shadow of a doubt from all those I've tried. I'd love to try IV DMT, or administered through an IV infusion. Phenethylamine/psychedelic amphetamines aren't a family I have experience with, yet, although I should be trying DOM and 2C-D, perhaps 2C-D-nBOMe, or similarly substituted benzylamines. Beta-keto-2C-B is the only one, methylenedioxy-ring-bearing entactogens excluded, that I have tried. NOT for snorting, fucking christ, never felt anything so painful to have up my nose, short only of worse pain coming from having accidentally had a blob of molten sodium metal covered in fused molten sodium hydroxide (caustic soda) go off with a bang after a tank of argon being used to protect an electrolysis cell gave out, empty, causing the Na metal to explode and shoot off through the air, only to by some tiny chance and Murphy's law, land up my right nostril, burning a temporary crater into my nasal septum that would make daniella westbrook proud , at least, before going up in flames after I blew it out of my hooter hole as fast and as hard as I could manage
Lol, I'm just glad that little incident didn't end up with that blob of NaOH-crusty molten sodium actually igniting, or exploding before I got it out of my nose in a fucking well hurry :/
Entactogens/empathogens aren't usually my thing, but I do really enjoy high dose insufflated AMT, n-ethyl-5-APB was nice too, although I liked the AMT more out of the two of them. 5-(N-CH3)-APB was good too, I like the fact that neither of those two 5-APB analogs possessed much in the way of stimulating properties. I credit AMT actually, with helping me largely overcome my PTSD.
Misc substances:
diethyl or diisopropyl ether, tizanidine and/or clonidine, both alpha2 adrenoreceptor agonists, and have proved to be a gift from above, in terms of dealing with, and suppresion of overloads, overstimulation, and comedowns, as well as sleep.
Valerian extract (Valeriana officianalis)..techically this contains allosteric GABAa positive modulators, and can be a sleep inducer, but that effect is relatively mild. Where Valerian really shines is when taken in high doses three quarters to one hour or so before going to bed. Then its about the most powerful and vibrant oneirogens I have ever encountered.
Nootropics? Pramiracetam, although I've high hopes for DM-235. Although I cannot afford it at the moment, galantamine, an anticholinesterase agent that was found lurking in snowdrop bulbs, a cholinesterase inhibitor with a difference, as it also activates the alpha7-type neuronal nicotinic acetylcholine receptor, increasing its nootropic effectiveness quite a bit, or it seemed to, compared with other anticholinesterases without the alpha7-NAChR agonism. IIRC this results in release of BDNF (brain-derived neurotrophic factor