If you're looking to play the energy sector and you want to invest in one of the market leaders in that space, then I would suggest taking a strong look at Marathon Oil ( MRO). This company, through its subsidiaries, engages in the exploration, refining, marketing, and transportation of liquid hydrocarbons, natural gas, crude oil, and other petroleum products worldwide. So far in 2011 this stock is leading the pack in the oil sector with shares up around 32%. Marathon Oil is the fifth largest refiner and marketer of petroleum products in the U.S. The company has a market cap of $34.94 billion and trades a very reasonable forward price-to-earnings of just 9.22. Marathon Oil isn't just benefiting from the trend of rising oil prices in 2011, but it's also reaping in big profits due to its refining business. The company just reported a very strong quarter that showed quarterly revenue of $20.2 billion which was up 26% from the previous year. Marathon's refining and marketing business segment brought in $213 million during the most recent quarter, which is a gigantic improvement from a loss of $18 million last year. From a technical standpoint, the stock has already started to breakout above some previous overhead resistance at around $47.15 a share. I expect this stock to continue to trend higher in 2011 and investors should look to add this name on any meaningful pullback. Richard Snow's Snow Capital decreased its position in Marathon Oil by 6% in the most-recent quarter but still holds 2.1 million shares; the stock comprises almost 3% of Snow Capital's total portfolio. According to Karvy Global, it's one of 10 energy stocks for rising crude as well as 10 energy stocks analysts favor, and with a B buy rating from TheStreet Ratings, it's one of the top-rated oil and gas stocks.