Jacksonville, Florida. Paper mills.
Gilroy, California.
Doesn’t smell like sewer or paper mill, but I am not a fan of the smell of garlic. Yuck.
A hot afternoon in Calexico, California, just across the border from Mexicali will make your hair fall out. The New River flows north from Mexicali on its way to the Salton Sea and has the honor of being the most polluted river of its size within the United States. Wiki link: New River (Mexico–United States) - Wikipedia
I’m glad someone else mentioned Richmond. That’s the first city that came to mind when I saw this thread, but didn’t want to post it because I’d only driven through and couldn’t rule out the possibility that my passenger had farted just as we reached Richmond.
I’ve protested all those people making fun of New Jersey, but it’s undeniably true that the chemical plants and oil refineries of Linden and Elizabeth in New Jersey (not Newark, although it’s not far from the Newark Airport) stink unbelievably bad.
It doesn’t help that, at night, with the flames burning off the effluent atop the refinery towers (like in the opening shot of Blade Runner), the place looks like a scene out of Dante’s Inferno. Or the “Hell” panel from Hieronymous Bosch’s Garden of Earthly Delights
I’ll add that my hometown in New Jersey has on occasion smelled really awful when the “sanitary landfill” cover not far away has been breached, and the wind’s blowing from that direction. Nowhere near as bad as the refineries, but still…
And it’s not my intense dislike of Rochester, N.Y.that makes me point out that, on days when they’re roasting the hops, the Genessee brewery makes downtown Rochester smell appalling.
Plains, Texas, has embedded itself forever in my olfactory memory. This shithole of a town reeks of petroleum. It permeates everything, there’s no getting away from it.
Oh, and Bangkok doesn’t ALWAYS smell like a rose. 
I also forgot to mention, that (although I’ve never detected a stink) the city of El Segundo, California is nicknamed (heh heh) “Smell Segundo” because of its sewage treatment plant.
I passed through Casper, Wyoming about ten years ago, and my main impression of it was that I’d be embarrassed to have people from out of town visit me if I lived there, due to the pervasive oil refinery smell. It may have just been a bad day, but it’s by far the worst offender I can remember, and I’ve been to New York, New Orleans, and Venice.
The mushroom farm in Oxnard, CA (might be Channel Islands, CA) smells worse than anything else I’ve ever smelled in my whore entire life.
Mumbai is certainly top of my list.
It hits you when you firstget off the plane, and steadily gets worse as you head to town. Amazingly, after a few weeks you become used to it.
Once you pass Omaha on Interstate 80 westbound, the entire state of Nebraska smells like cow shit.
This pretty much exactly what I was going to say (besides the vomiting part). I was stuck in traffic right near to the slum area, and I had my nose pinched the whole time, and couldn’t escape. If hell had a smell, this would be it. I’ve smelt many of the cities mentioned here, but nothing compares to the slum section of Mumbai.
You should try the Mapocho river, in Santiago de Chile, it has a stench that will melt the hairs of your nostrils.
Or at least it was that bad when I visited the place back in '98.
What?
Not a mention of Cairo
Augusta GA. Loaded with chemical factories and a paper plant. Nothing like the smell of chemicals in the morning.
I used to deliver pizza in Longmont, CO. There were a couple of neighborhoods we had to go around the stinky side of the turkey processing plant to get to. The big challenge was to see if I could hold my breath for the duration.
Third post! Now that’s smelly. 
I could sometimes smell it in Little Rock when I was a child.
Smells more like a paper mill to me. Some days, it’s pretty pervasive; other days, the air smells sweet and clean.
Nampa, Idaho has a sugar beet processing plant about a half mile north of I-84 exit 36. It’s a very distinct (or shall I say, dis-stinked) odor. It’s not quite so sulphuric of smell as a paper mill produces (I’ve also been through Lewiston, which was mentioned upthread), but a sort of bitter, mildly acrid sort of smell, somewhat resembling the smell of old potato peels.
When I was a kid, Burlington, Vermont, used to smell of maple. There was a plant on the waterfront producing Maltex, a maple-flavored hot cereal. It smelled wonderfully.
(They didn’t actually use maple syrup - they used a far less expensive flavoring from a spice called fenugreek. It didn’t matter.)