The most famous parental favoritism is probably between Monica and Ross from FRIENDS. It was really heartbreaking to see their parents go above and beyond for Ross, while Monica got the bare minimum from them. That was just fiction, but it happens many times in real life.
Just look at this couple who were always helpful towards their elder daughter, but expected the younger one to do her own chores. One day, she finally snapped and called them out for this differential treatment. Here’s how they tried to dramatically justify their toxic behavior…
More info: Reddit