It makes sense that some investors like to closely follow which stocks billionaires own. After all, those billionaires didn't become wealthy by making poor investment decisions. But, more often than not, ultrarich investors don't agree about the best stocks to hold. As a case in point, Warren Buffett, Bill Ackman, and David Tepper have portfolios that look very different. But there's one stock -- and only one stock -- that all three of these billionaires own. The sole common denominator I won't try to keep ...