Hm. How to pick?
I can think of a few that tie for top place.
1- I was brand new to dc so went above and beyond for mom. When I stopped doing that, dcm pulled dcg, refused to pay me and then reported me. Left threatening phone messages for a few weeks.
2- Parents pushed me on every single policy I had. Then acted like I was the bad guy for enforcing them. Multiple suspected dope and drops, picking up late, arguing everything, bullying behaviour from dad, bullying behaviour from mom. They caused me a lot of stress while they were here. Come tax time, they started sending harassing emails about receipts (which I had given them). Rather than asking for replacements, mom tried threatening me with government action if I didn't comply. Um. Ok. They were only here a few months before I termed them but it felt like a life time.
3- Clients paid a deposit to hold a spot. Changed her mind 2 weeks later. Repeatedly harassed me to get her money back. I refused. She cursed me up and down. Turns out, she acted even worse for the provider that ended up with them (termed them after 4 weeks). She was a pumpkin.