Tough call!
I'd have to say the runner up would be the heroin addict couple. At first it was very subtle, long hours locked on their room, a strange smell. But slowly it became a hardcore addiction: unrealistic plans like farming but coins in our apartment and opening a massage parlor. Regularly wetting the bed.
But they were just a warm up act to Greg.
The first night he moved in, in an attempt to seem more spontaneous to his then girlfriend, he spent the night breaking glass bottles inside. Warning sign number one: unable to have respect for ones environment.
This was quashed but briefly later he'd start sweeping garbage to my door. When confronted he would blamed the other room mates. Warning sign 2: doesn't take responsibility for ones actions.
One day he locked himself out of his room. Solution: break down the door. Then spent months complaining his door wouldn't latch properly. Warning sign: lack of forethought.
But probably the worse incident with him was the Halloween where his ego was bruised. Supposedly he punched a guy in a Nazi costume that night (which I think we can all totally support) but ran into Nazi douche later and got his ass kicked. He came home staring punching walls, breaking bottles and drinking heavily. Later one of his girlfriend came over. Can't tell you what exactly happened on his room but when I heard her screaming I rushed to his door and confronted him wearing only a towel ( having been sleeping with my girlfriend at the time and a big fan of Douglas Adams). He saw red and started challenging me to fight. Now I'm not going to aggravate the situation by hitting some drunk range filled retard but neither can I have him making girls scream, cry and god knows what else. At this point heads of the other room mates are poking out into the hall. He fakes a punch and my hands fly from the towel leaving me naked and him fuming. Can't really remember what happened next but somehow one of the room mates convinced him to watch TV in the kitchen while the girlfriend makes her exit. Warning sign: I'll let you make up the lesson.