What Holden hates most of all is Phonies.
Phonies are people who are not genuine, who are only pretending to be who they seem to be. In Holden’s case, they are most often adults and he tends to see all adults as Phonies.
One of the biggest reasons I left Elkton Hills was because I was surrounded by phonies. That's all. They were coming in the goddam window. (ch 1)
Holden goes on to describe how the Headmaster at his old school would pretend to be polite to everybody’s parents, but if a student had funny-looking parents, he would pretend to shake hands and enjoy their company, and move on to someone else.
I can't stand that stuff. It drives me crazy. It makes me so depressed I go crazy. (ch 1)
To Holden, the world is full of Phonies. He does not know who he can trust. He feels like he has been betrayed by everyone around him. Adults, who are supposed to be genuine and protect him, are the worst offenders. As a result, Holden becomes depressed and frustrated.
Unfortunately for Holden, there are a lot of Phonies in New York. He can't escape. He feels trapped in an artificial world, looking for something real.