depend on how you can handle your drugs. i used to get coked up, take quaaludes, drop couple strong rolls, drop acid, get drunk, and smoke heroin in the bathroom(not all at the same time) in high school and i could keep my shit together.
then me and a friend did a few lines in the bathroom one day and he threw his shoe at someone in class who disagreed with him over some minor thing.
now i can take a hit of acid and the only way you could tell i'm tripping is my pupil size(i have light green eyes). but i can easily manage going in public on amphetamine pills, benzos, oxys, and while pretty drunk.
on PCP, ketamine, or large doses of DXM i can have i tough time acting normal(i tend to wander with a blank stare and have poor cordination). also on crystal meth i look like escaped from a mental hospital and i tend to be overly talkative and act a bit "wound up" so ice isn't the best choice of drug to be on in public.
anyways what the strongest drug you can keep your shit together on in public varies greatly from person to person- it can be a weak to moderate strength opiate or a powerful hallucinogen like LSD.