The worst accomodation I had was a flat share in tooting. I was allergic to the carpets and one of my flatmates punched a hole in the bathroom wall in frustration so you could see people sitting on the toilet when cooking in the kitchen.
But several things made the place tough to live in. One of my flatmates was a guns and roses fan with a drumkit and who did shift work. Another (and a very close friend) was a schizophrenic who earned extra money as a male prostitutes. You have no idea how depressing it is coming home and finding the living room floor covered in vaseline. He was lazy and rather than come downstairs to the toilet he would steal kitchen pots and use them as chamber pots.
I once caught a stray dog which had gotten in, going through the old KFC boxes which littered the living room looking for food.
Finally the drummer, had a lot of friends who were smack addicts. They would come round and shoot up where-ever they liked. If you complained they would threaten you.
I eventually decided to get my own place .
The worst place I ever slept the night was a park in Athens. Disgusting night, with loads of mosquito bites. The second worst was the beach in Sharm-el Sheik before it was developed. We were lucky, nothing happened to us, but I remember meeting some scandinavian girls who complained that someone had ejaculated on them while they were sleeping, and then stolen their money.
It can always get worse.