It has been a good year so far for stocks, with the S&P 500 up 15% since the beginning of 2017. But that's just an average of the 500 stocks on the large-cap index; some have done even better.At the top is Vertex Pharmaceuticals (NASDAQ: VRTX). It is the best-performing stock on the S&P 500 this year, boasting gains of 110%.Data source: YCharts.com.