This is easy...Atlas Shrugged
It is a treatise on selfishness. A stench of arrogance leaks out from every paragraph. It showed me the worst of the worst. What not to be. I like to think reading it made me a more empathetic, social, caring person. It also awakened my antenna to the vileness in people that may lie just below the skin.
That is was excruciatingly badly written was a bonus. It showed me that dangerous ideologies are persistent enough to survive laughably poor communication.
I thought it was well written though. She just has a weird style; I kinda dug it. I would call it writing Brutalism, and it was very clear and to the point.
I used Fountainhead to introduce my kids to Ayn Rand and as a warm up to Atlas Shrugged.