I'm not sure about the world, but the worst smell I've personally experienced was a grease-trap that hadn't been cleaned or indeed opened in about 6 years. A busy kitchen's accumulation of grease, blood, juices, and othere nastiness, just building up over time. My crew and I opened it, and two guys vomited immediately, I gagged abit, and someone at the other end of the restaurant in the bar said they felt sick, and what the hell was that stench. If you've ever had to open even a relatively clean trap on a regular basis, you know what I'm talking about.