The worst guests I ever heard of stayed with a former boss of mine. It was her husband's brother and his wife and two kids.
This was in the US, so of course, my boss 'Sandra' could only get 3 days off for their 2-week long visit.
During which time she and her husband, both of whom worked full-time and had 3 daughters themselves, did all cooking, cleaning, shopping, etc. The BIL never put his hand in his pocket or said thanks.
The creme de la creme came one night when the BIL's kids were throwing up all over the place. His wife went to help Sandra and BIL stepped in and said, 'Don't, Terry, you're on vacation.'
At that point, Sandra's husband let his brother know the holiday was over.
Oh, and a dear friend of mine has a son who is married to a bit of a skank.
They came to Scotland from America with her 9-year-old son from a former relationship and a 9 month old daughter they had together.
The gal and her son did nowt but complain and the gal left the baby's poop nappies all over the floor, wrecked the house (and my friend's a neat nick), and then asked her for money.
They never came back to stay with them again!