It's impossible to talk about billionaire investors without bringing up Warren Buffett. Known as the "Oracle of Omaha," Buffett is considered by many to be the world's best investor. Forbes ranks him as the second-wealthiest man in the U.S. (after Microsoft founder Bill Gates), and the richest investor with a fortune valued at $47 billion in 2010.
Through his storied firm Berkshire Hathaway (NYSE:BRK.A,BRK.B), Buffett has made some major moves in the last year, most notably the acquisition of Burlington Northern, which ceased trading earlier in 2010. But the railroad giant isn't the only big purchase Buffett has under his belt.
While Buffett didn't initiate any new positions in Berkshire's portfolio in Q1, he did up the stakes in a few of the company's portfolio positions, including waste management firm Republic Services (NYSE:RSG), document management company Iron Mountain (NYSE:IRM) and medical supply giant Becton, Dickinson (NYSE:BDX).
Berkshire Hathaway also closed positions in a handful of companies, including WellPoint (NYSE:WLP) and UnitedHealth Group (NYSE:UNH), two major health insurers. (For more picks, see A Glimpse Into Buffett's Portfolio.)