Nepal is pretty funky in that regard, knew a couple who went there last year, the wife is a stickler for cleanliness and they came back saying it was the worst vacation they'd experienced.
The wife is a teacher and probably read about the history and religions there, it is interesting in terms of that but basic sanitation is lacking in most of the hotels and rivers reeking with sewage and trash is not acceptable. I think she's learned to research her trips better now. They took another one a few weeks later probably to make up for that failed experience.
Worst developed area? Probably India or Pakistan.
Worst non-developed area? Probably somewhere in Africa.
there are some pretty bad places but i can only speak of the ones I've been to Olongapo city in the Philippines was terrible and Mexico is horrible and makes sense of why everyone wants to leave
It also always depends where in that country. In the country side its often worse.
And even if sanitation facilities are not too bad, there are many countries where for example all the sewage stuff goes unfiltered into rivers or the ocean. Even many so called rich countries do that.