Warren Buffett is arguably the most successful investor in history, so tracking his buying activity could be a great way to discover new investment ideas for your own portfolio. Fortunately, large investors like Buffett have to file a 13F report quarterly with the SEC detailing what they bought and sold in the prior quarter. In the case of Buffett, Berkshire Hathaway‘s (NYSE: BRK-A) (NYSE: BRK-B) most recent 13F suggests investors might want to consider buying Visa (NYSE: V), Phillips 66 (NYSE: PSX), and airline stocks.