This chart shows how Valero and Marathon Petroleum, the largest US refiners, performed so far this year. In fact, refiners are the best performing energy stocks this year. Here's why.These companies take crude oil and process them to make fuels such as gasoline and diesel. They've done better than any other group of oil and gas sector companies. Refiners are doing well because the price of fuel has gone up a lot more than crude oil, which is their feed stock.Oil prices have gone up about 25% since the confl ...