Any kind of hallucinogen if you take a high enough dose will probably be the most impairing.
Besides those though, its got to be alcohol.
I mean ... anything though if you dose high enough it will leave you largely fucked up (opiates, benzos, whatever you can think of really). But... none of them really combine all the elements of fuckedupness like alcohol does. It fucks you up mentally, physically, its fucks up your speech, your memory, very toxic, will make you feel like shit the next day. So alcohol wins.