Man of Distinction

Aliens, looking for slaves, pick up Al Hanley as a specimen to evaluate the fitness of humans. But Al is a drunkard and convince that the aliens are hallucinations, and he in turn convinces the aliens that humans are worthless. Abandoning plans to take the Earth by force as being not worth the effort, they take Al back to a zoo and keep him well-supplied with booze (which their tests showed to be an important part of his metabolism), where he continues to protect the Earth to this day.