Without a doubt - our neighbors in our old duplex in Lafayette.

They would get drunk every night and then fight - screaming things like "why don't you go ahead and hit me" and slamming doors. Our walls would rattle. The worst time was when they did this on Christmas Eve.

They had two litter boxes that they kept in rotation so one was always sitting at our shared front door in various stages of cleanliness. He would dump the used kitty litter over the balcony onto the herb garden below.

He would accept our UPS packages (he didn't have a job) and then forget about them. They would show up inside our garage weeks later.

He never wore a shirt and would sit on the balcony right next to us so that he was always the view from our living room. If he looked like Brad Pitt I wouldn't have minded. He didn't look like Brad Pitt.

Their cats tested positive for feline leukemia but they still let them roam the neighborhood. They often came over the rail to our balcony to share in the water bowl we kept outside for our cat. It was weeks before I knew this. I was livid that their carelessness put my elderly cat at risk.

I could go on. There's lots more.

Moving away from them was one of the happiest days of my life.

Probably my neighbor who screamed obscenities at her children (ages 4 and 11), which came through the thin walls of our converted Victorian. My roommate called CPS on them and we heard her loudly accuse me(!) of it. When she confronted me about sleeping with her husband, whom I'd never met (she made her small daughter "confirm" my identity), I'm 99% sure she put a hex on me.

Hooray for the plentiful supply of rentals in SF!

How about a Superintendent from hell?
I have one that is a rat fink and will tell the co-op board about any little infraction.
Like if he notices that you have overnight guests he'll accuse you of subletting. Or if you use a large room divider (which is not glued, nailed or adhered to the real wall) he'll accuse you of building a real wall.
He is rude and rarely comes to do repairs.
Even the neighboring Supers hate his guts because he yells at them telling them how to do their jobs.
This guy is a real whack job btu the co-op board keeps him because he's a rat fink. Oh well.

I've had plenty... But here are two that stand out:
The neighbor who kept four little anklebiters in his side yard. They had been de-barked but still had that annoying hoarse yap -- all day long. He also insisted that each apartment take up one space -- on a public street.
Then there were the two Goth party girls who entertained scary drunk skinheads all night. They lived above me and apparently their boyfriends enjoyed chasing their cat around the apartment. One morning at about 4 a.m. I had had enough of the loud music and went up to complain. They weren't even there. My friend and I broke in and turned the music off. Then they didn't come back for two days, leaving their cat trapped without food and water. We broke back in and rescued the cat -- but unfortunately they saw him in my window when they finally came home and took him back.

I once had an upstairs neighbor family I called the hippo ballet and bowling alley, for reasons that should be obvious.

The landlord , however, thought I was being unreasonable.

Until said family skipped out in the middle of the night and he had to clean out the festering swamp they left and found..you guessed it..several bowling balls that they left. The damage that the bowling balls had left on the floors, walls and appliances was spectacular and considering the damage to my nerves and sleep habits that I had endured for several months my sympathy for the landlord was limited to bringing him a box of tissues. And my notice that I was leaving at the end of the lease.

I had a downstairs neighbor who hoarded things (junk), never cleaned her apartment because it was full of stuff, had two dogs, two cats and a bunch of rats in cages. It STUNK so bad. I almost gagged every time I came home to our communal entry. She would buy cheap perfume and spay it out in the entry to try and cover up the stench. The smell permeated my apartment too. Finally, with the help of the condo board we got her out of there. When she left the realtor had to have a contractor come in and tear out the kitchen (cat poop inside the cabinets) and all the flooring, baseboards, doors, door trim and DRYWALL because it was covered in animal feces. Even the dining chandelier had cat crap on it because the stuff was piled that high in the room! It cost over $10,000 to make it habitable again.

My first apartment was next to a bunch of drunken college students who were on the four-party-a-week system. My place acted as some kind of echo chamber for their godawful music.

The basement was supposed to be shared, but they took it over. When I complained to the landlord and she asked them to tone it down, they said that I was more than welcome to join them.

I'm still mad at myself for never having the nerve to call the cops. Though they did come around asking about an apparent assault that had taken place on our shared porch - fortunately I'd been out of town that weekend.

I had a neighbor who would come over, knock on the door, and invite himself in for talks every once in a while. One day he came over to tell me something and asked if he could use my bathroom -- mind you this person lived 2 apartments down from me and was a man in his fifties, while I'm a woman in my early twenties. I said yes, reluctantly, and a couple of minutes later he came out of the bathroom and said "I only peed, so I didn't flush." I told him to go back in the bathroom and flush immediately.
He was generally a rather strange guy, and was later evicted for pointing a handgun at the landlord.

I'll stick to one group. I live in a large coop community for a number of years. Throughout the years, I have experienced at the beginning hospitable civilized neighbors (unfortunately for me they left) and replaced with nosey opinionated control freeks. Things have gotten to the point where the coop building association have floor monitors to report back to the building captains who then place neighbors on a watch list for any suspicious activity -- such as have a job that requires one to work after 5pm and come in the morning, walking "excessively" on the floor, grilling on own terrace/balcony, growing a garden on own terrace/balcony (herb plants are often mistaken as marajuana with neighbor on the phone calling the building captain al the while waving at us and afterwards we receive a "friendly" visit from the coop representatives. )

I guess the fact that we have been living here in harmony with our neighbors long before these anti-social neighbors showed up just doesn't seem to click with the central coop board. However, I'm not alone, our community newspaper is full of in-fighting between neighbors, and community board members. At this point, I'm looking for greener and saner pastures to finally nest.