Writing for The New York Times, Randall Monroe (creator of the xkcd comic) looks into the world's worst smell. It turns out that there is no universal agreement, but a strong contender for the title is something called U.S. Government Standard Bathroom Malodor, "a substance that was designed to mimic the scent of military field latrines, in order to test cleaning products."
Another contender was accidentally cooked up one day by Derek Lowe, an industrial chemist:
Dr. Lowe said that the worst thing he'd ever smelled in his career as a chemist arose when he inadvertently combined dimethyl sulfide (think farts) with some silicon he was putting through a reaction called a Peterson olefination. Neither odor would smell great on its own, but combined they produced something transcendently foul. "It smelled like what you'd imagine the exhaust of a U.F.O. to smell like," he said. "It was spectacularly weird and horrible."