I had expiremented with moderate/recomended doses of lsd and psilocybin before participating in an auahuasca retreat... and the doses were shocking.
This was a relatively tame centre in western Europe that had trained psych nurses in attendance. Still, the Shaman handed out monster doses... and offered a second one a couple of hours in... and again the following day.
Many traditional practices conceive of "levels" corresponding to doses, and the lower levels are not the ones associated with transformative spiritual experience.
I have no recomendations of my own.
That's what drug prohibitionists would love to happen, so they should encourage such a "wrong" experiment instead of prosecuting it. :-)
Unexpected things happen, mental states swing, and now you're in a mentally compromised state in-between reality.
There are some other commenters on here that seem to implicitly be talking about what I'd consider hero doses. I don't need high doses to experience notable differences in my ability to sense and self-reflect.
Chemically and physically, LSD for example is gone in a few days and certainly weeks. But taking two days in a row needs "insane" amounts to get you the same (physical, chemical) effect. MDMA is almost entirely gone in months, after which a normal dose will get the same (physical, chemical) effect. Good documentation with even calculators for this can be found online (be aware that it may not be allowed to even visit such sites in some countries!)
I deliberately emphasis the "physical and chemical" because the experience itself may be something you get used to. First time MDMA is wildly different from any time thereafter for many people. For many this is even stronger with LSD or Psilocybin. Many people report that the (first) experience opened up their mind and being profoundly and lasting. The more one gets used to the feeling the more this feeling itself becomes routine.
The intense love and euphoria from e.g. MDMA may be the same on an objective level, after letting the chemicals disappear over months, so technically one can achieve this level a few times a year. But the feeling itself is something one could get "used to" and "become bored by". Similar to how experiencing a rave sober for decades, or even how after decades of traveling it could get boring and a routine.
My point is: indeed first times are important. Even if the textbooks say that tolerance is gone after X time.
You just open the door for many more first times to come
Now, an important factor here is that a first time often comes with anxiety, insecurity, and even fear. With many substances these feelings get amplified so that makes the entire experience be mediocre or even bad. Especially for that, I think it's very important to experience this in a safe, loving, caring and depending on the type taken, controllable, silent, busy, energetic, dark, light etc environment.
MDMA comes to mind. I've also heard (though not tried, nor will I) the harder narcotics such as heroine and meth are almost never as good as the first time.
The first time being the best time is often exactly why people become addicts.
Not to downplay all obvious and less obvious hazards of these 'harder' narcotics, just saying the whole 'first time being the best time' as an explanation of why exactly people become addicts relies very much on hearsay. It's the sort of language that came with the whole war-on-drugs-thing, and does not reflect reality.