I had terrible roommates all throughout my university career but two of them were the absolute worst.
My friend moved into our 5 person house with his wife who he met on the internet from across the globe and they "accidentally" got pregnant. She was kind of a neat freak before but after she gave birth it was a whole new level of neat freak. Keep in mind that this was a house meant for 5 people and there were 7 people (including the baby) living there. She became obsessed with having the house completely spotless and if we kept our dishes in the sink for more than 1 hour, she would start to scream at us. She had a fit when someone left a piece of broccoli in the kitchen sink but she found it acceptable to wash her baby's ass in the sink. It got so bad that none of us wanted to cook in the kitchen. She treated the house like it was hers that we were living in rather than a house we were renting as mutuals.
Not my roommate but my boyfriend's roommate was a horror. My friend (from the first story) and my bf were always the ones to take out the garbage in the kitchen and bathrooms but it got so bad that they started to keep their garbage in their rooms. Their one roommate got pissed that they weren't taking out the garbage anymore, took the garbage in the kitchen and threw it at my friend's room door. He also never did his dishes and once they ran out he started using other people's dishes until my bf and friend had to keep their dishes in their rooms. His mom eventually came and did his dishes for him after 3 months.