What is a smell that you absolutely cannot stand?

The smell of tripe cooking is truly gag-worthy.... I wish I had a barfing emoji.

Any kind of disgusting smell... If I had to choose one, I'd choose the smell from a sewer

Rotting meat.  Yuck!

bad cat litter that needs to be cleaned.

While all these ⬆⬆⬆⬆🙊 other answers are very ummmm appropriate to say... I'm gonna add something that was a first for me recently.... A ran over skunk in the middle of the road and no one wanting/willing to pick it up for a few days! (Shudders) you can still smell the spot driving by even though the body is gone now! 

Oh and boiling/cooking ribs! YUCKETH!!!

I agree with the observations about sewage. But oddly the smell that most makes me want to barf is that overpowering sickly sweet faux "perfume" that hits you when you enter a garden centre or similar and they are selling those Yankee candle things. I now avoid shopping at two such local centres. How's that for sales prevention?

The paper mill.

Artificial Raspberry flavoring in anything.