Bill Ackman is one of the most widely followed investment managers in the world. He's in charge of Pershing Square Capital, a hedge fund focused on investing in just a handful of Ackman's best ideas. His highly concentrated portfolio is full of great companies, but he might have just made one stock the biggest holding at Pershing Square.

Last week, the stock of Uber Technologies went on a wild ride, falling hard after Uber reported earnings on Feb. 5, only to rocket higher toward the end of the week. On earnings, while revenue and earnings per share actually beat expectations,

Uber stock hit its highest level since October after Ackman disclosed his Pershing Square hedge fund accumulated a 1.4% stake in the ride hailer.