Final answer:
Physostigmine is the most likely medication to reverse the patient's delirium caused by an overdose of an antihistamine like Benadryl due to its property of being a cholinesterase inhibitor that can cross the blood-brain barrier.
Step-by-step explanation:
The medication most likely to reverse the patient's delirium, which is suspected to be due to an overdose of a generic form of Benadryl, is physostigmine. Benadryl is an antihistamine with anticholinergic properties, and physostigmine is a cholinesterase inhibitor that can cross the blood-brain barrier, thus counteracting the anticholinergic effects. Options such as atropine and scopolamine are themselves anticholinergics and would worsen the condition, while naloxone is an opioid antagonist, and flumazenil is a benzodiazepine antagonist, which are not appropriate for this scenario. Pralidoxime (2-PAM) is used to treat nerve agent poisoning and would also not be suitable here.