I have a top 3.
First house: Sister of a friend, was invited to a birthday party for child of said sister. House, it was actually an second floor apartment, was so nasty I refused to use their bathroom. I sat in one spot and tried not to move too much.
Second house: I used to babysit for an agency and got a job babysitting 2 kids. Went to the house. Noticed upon entering house the distinct odor of cat urine, also noticed that the furniture and carpeting looked kinda scuzzy - like no one have ever dusted or vacuumed. Talked to the mother about the care of the kids. The mother shows me the kitchen, odor of cat even stronger. Come to find out they were leaving open cans of cat food out and were using a lower cabinet - it looked horrible. The mother then shows me the frig - omg it was disgusting - it looked like they were letting the cats use the frig! It smelled like cat too!! The mother then tell me I can eat anything in the frig. I order a pizza.
Third house: Working with my parents (house cleaning business). We go to a house of a client or I should say 2 clients, both doctors. They live in an expensive 3 level house hanging off a hill. We walking in the house and get hit with the smell of cat. They have 3 cats, who have total run of the house. I should also say they had a nanny, she was big and her only job was to take care of their kid. I think the kid was about 2 years old, maybe younger. The kitchen was filthy, dishes piled up since the last time my folks had come. Cat food all over the floor. My dad had to get something from their garage and I went out there with him. They had been letting the cats poop in the garage - cat poop all over the place. They even had unopened cans of beer sitting out there with the cat poop! But the worst part was the master bathroom. It was carpeted and they had two cat litter trays that were overflowing and the cats were pooping and pissing on the carpet!!! They are doctors!!!