Many investors are anxious these days, with the stock market going topsy-turvy. I doubt Warren Buffett is one of them, though. The legendary investor is worth billions more now than he was at the beginning of the year, with Berkshire Hathaway (BRK.A 0.82%) (BRK.B 0.39%) stock up nearly 16% year to date.However, some of Berkshire's holdings have performed even better than that. Here are Buffett's three best-performing stocks so far this year -- and whether they're good picks to buy now.1. BYDShares of BYD (B ...