Etorphine was the first potent opiate agonist employed primarily for use in non-domestic and wild species. Etorphine was 500 times as potent as morphine, with a very rapid onset and short duration of action. In morphine-dependent subjects, etorphine suppressed abstinence but for a shorter period than morphine. Etorphine is a full opiate agonist and binds to multiple opiate sites in the central nervous system. It has a potent effect on depressing the respiratory centers of the CNS thus resulting in apnea being commonly seen in immobilized animals. Etorphine revolutionized the ability of biologists and veterinarians to safely capture and restrain many species that previously could not be handled. Etorphine is not currently commercially available due to lack of production by the manufacturer.

Etorphine:
https://drugs.ncats.io/drug/42M2Y6NU9O

Etorphine is type of opioid analgesic that is semi-synthetic. Unlike other opioid drugs that are prepared from the substance that generally occurs in the opium poppy extract, Etorphine was originally prepared from oripavine, or the poppy straw in related opioid plants, Papaver bracteatum and Papaver orientale.

The only current legal uses for Etorphine are largely for veterinary purposes, commonly to immobilize large mammals, such as elephants and rhinoceroses. The potency of Etorphine is approximately 1000 times that of morphine and is typically given to mammals via a projectile syringe dart. The drug Etorphine, although difficult to obtain and possess, can become a misused substance if in the wrong hands.

Etorphine was the first potent opiate agonist employed primarily for use in non-domestic and wild species. Prior to its development and registration in the early 1960s, the only immobilization tools available were succinylcholine, nicotine sulphate, and ropes. Etorphine revolutionized the ability of biologists and veterinarians to safely capture and restrain many species that previously could not be handled. Its reversibility with diprenorphine (M5050), a potent opiate antagonist, made it a significant advance in zoo and wildlife medicine. By virtue of its long history in veterinary medicine and wildlife biology, etorphine use in numerous species is well documented in the literature. Although newer and more desirable opiates have been developed (carfentanil, A3080), etorphine is still the drug of choice for exotic equids, rhino, and other hoof stock.

Etorphine's potent opiate agonist activity produces rapid reversible immobilization at low dose rates. It has been used in almost every wild and free-ranging group with the exception of members of the family Felidae which have severe and adverse reactions to all opiate agonists. In the United States, it has been extensively used in feral horses, deer, elk, bison and, to some extent, in bear in years past. Its availability in the late 1960s concurrent with the development of reliable dart delivery systems allowed wildlife managers and researchers to conduct projects and studies that previously were impossible In zoological collections it has been used in elephant, rhino and members of the family Equidae. Its use in other species in zoos has been replaced by carfentanil, medetomidine combinations, and Telazol in combination with alpha-two agonists (xylazine, medetomidine).

A Reddit Discussion:
As far as non-recreational, looks like Etorphine is the most potent. 25mg to take out a black rhinoceres. It is so potent it can't be used in humans. Perhaps in could be used in picograms if one was really determined. Something like this would, naturally, have to be done a controlled environment with professionals

Etorphine:

Etorphine is available legally only for veterinary use and is strictly governed by law. It is often used to immobilize elephants and other large mammals.

• A semi-synthetic opioid
• An analgesic potency approximately 1,000 - 3,000 times that of morphine
• First prepared from oripavine, it can also be produced from thebaine

It was first prepared in 1960 from oripavine, which does not generally occur in opium poppy extract but rather the related plants Papaver orientale and Papaver bracteatum. It was later reproduced in 1963 by a research group.

Veterinary-strength etorphine is fatal to humans. For this reason the package as supplied to vets always includes the human antidote along with the etorphine (antidote is generally naloxone)

M-99 (or M99) is the shortened term for the tranquilizer known as Etorphine Hydrochloride. It is used commonly as an animal tranquilizer and is known to be 1000 times stronger than morphine. This drug is commonly used in the Showtime series Dexter:

Dexter Morgan: What am I even looking for?

Vince Masuka: Wait for it.

Dexter Morgan: This is a prelim. I don't have time to...

Vince Masuka: M-Fucking-99 (M-99)

Dexter Morgan: Etorphine Hydrochloride?

Vince Masuka: An animal tranquilizer more powerful than morphine. Causes total paralysis. That mark on her neck kept bugging me, so I ordered up a tox screen.

Dexter Morgan: (voice over) That's it. No more doughnuts for Masuka.