Jakarta (Indonesia)
I've been to many shitholes but Jakarta has a special place on my shit-list.
Tropical climate mixed with off-the-roof (air) pollution, laughable public transport, slums (really bad ones), probably the worst traffic (and drivers, sorry folks) in the world, corruption, mosquitos with dengue, diesel(!) power planst etc etc
I will never forget the constant smell of overused deep-frying oil mixed with exhaust gases.
plus sides: good and cheap food, some cultural sights and I freaking LOVED the harbour district and it's offerings.
Although the water was so filled with trash that I could literally walk from one boat to another without even getting wet.