Not the worst smell I've smelled, but the all around most disgusting bad smell: lived in an old farm tenant house for a few years. No public water, no well, got runoff from a barn roof to a cistern. I didn't drink it, but I did shower and brush my teeth with it. Start noticing a funny smell when I was showering and brushing my teeth. It had been a dry summer so I knew the cistern must be getting low. Popped the top off it and it was almost empty, with a bloated dead rat floating in it.
