Billionaire investor Bill Ackman is best known for his large stakes in companies like Chipotle (NYSE: CMG) and Restaurant Brands International (NYSE: QSR), but there's one stock in the portfolio of Ackman's Pershing Square hedge fund that could be the most interesting investment of all. I'm talking about Howard Hughes Holdings (NYSE: HHH), which is one of the most unique real estate companies in the market. Pershing Square owns about 18.8 million shares worth $1.6 billion at the current price, which gives Ackman and his investors a 37% stake in Howard Hughes.