It only makes sense that a great place to look for investment ideas is in the portfolios of market beating investors. Without question, Bill Ackman of Pershing Square is one of those great investors.
What makes Ackman's stock holdings an exceptionally great place to look for ideas is that he runs a very concentrated portfolio. That means that when Ackman takes a position he is risking a big percentage of his investor's capital. Therefore every idea has to be both exceptionally good, and pretty low risk and is likely being analyzed more thoroughly than stocks in the portfolio of a more diversified investor.