I've got a few:
My parent's house. So much dog and cat piss everywhere that everything made of wood on the floor is completely rotten. Chair legs, baseboards, hard wood floors. The basement has so much cat piss in the carpet that the floor is grey of mold. Also many leaks that's rotting away the ceiling and getting mold everywhere. The place reeks but they're OK with the situation.
A trap house. Id be more forgiving if it was just a place to crash, but the guy lived there. I went to buy, and it truly looked like something off of hoarders. Just crap everywhere, moldy cups, old pizza, roaches, no lighting, broken A/C so the Georgia humidity made that place stuffy and smelly. The bathroom was clogged and people were throwing their toilet paper in the trash can, which was piling over.
On a more mild note, a guy I worked with never showered. I have no idea how he never got written up for it, but his presence would stink up the entire room. His fingers left grease everywhere like he's just rubbed olive oil on them or something. Combined with the fact he wore what appeared to be the same shirt everyday. He told one of my coworkers that "soap doesn't really do anything for me, personally."