NEW YORK (TheStreet) -- Warren Buffett recently sold his ConocoPhillips (COP - Get Report) stock. The company was suffering and Buffett had lost money on his investment. But that doesn't mean all investments in the energy sector are bad ones.

Is Buffett being fearful when he should be greedy

Oil is down to around $50 per barrel after being over $100 for years, and oil stocks have taken a hit. That's why now could be a good time to buy, because, if the commodity recovers, oil stocks will do very well.

Also, refineries in particular do very well when the price of crude is low. Low prices means they can refine more oil, so, apart from the initial hit on inventory, future profit opportunities are strong.

What are the best mid-cap energy companies investors should be buying? Here are the top three, according to TheStreet Ratings, TheStreet's proprietary ratings tool.

TheStreet Ratings projects a stock's total return potential over a 12-month period including both price appreciation and dividends. Based on 32 major data points, TheStreet Ratings uses a quantitative approach to rating over 4,300 stocks to predict return potential for the next year. The model is both objective, using elements such as volatility of past operating revenues, financial strength, and company cash flows, and subjective, including expected equities market returns, future interest rates, implied industry outlook and forecasted company earnings.

Buying an S&P 500 stock that TheStreet Ratings rated a "buy" yielded a 16.56% return in 2014 beating the S&P 500 Total Return Index by 304 basis points. Buying a Russell 2000 stock that TheStreet Ratings rated a "buy" yielded a 9.5% return in 2014, beating the Russell 2000 index, including dividends reinvested, by 460 basis points last year.

Check out which three energy companies made the list. And when you're done be sure to read about which large-cap bank stocks to add to your portfolio right now. Year-to-date returns are based on April 17, 2015 closing pricesThe highest-rated stock appears last -- read more to see which one is No. 1.

TCP ChartTCP data by YCharts
3. TC PipeLines, LP (TCP - Get Report)

Rating: Buy, A-
Market Cap: $4.03 billion
Year-to-date return: -10.9%

TC PipeLines, LP acquires, owns, and participates in the management of energy infrastructure businesses in North America.

"We rate TC PIPELINES LP (TCP) a BUY. This is based on the convergence of positive investment measures, which should help this stock outperform the majority of stocks that we rate. The company's strengths can be seen in multiple areas, such as its growth in earnings per share, increase in net income, expanding profit margins, solid stock price performance and notable return on equity. We feel these strengths outweigh the fact that the company has had generally high debt management risk by most measures that we evaluated."

Highlights from the analysis by TheStreet Ratings Team goes as follows:

  • TC PIPELINES LP has improved earnings per share by 12.7% in the most recent quarter compared to the same quarter a year ago. The company has demonstrated a pattern of positive earnings per share growth over the past year. We feel that this trend should continue. During the past fiscal year, TC PIPELINES LP increased its bottom line by earning $2.67 versus $2.13 in the prior year. This year, the market expects an improvement in earnings ($2.75 versus $2.67).
  • The net income growth from the same quarter one year ago has significantly exceeded that of the S&P 500 and the Oil, Gas & Consumable Fuels industry. The net income increased by 14.6% when compared to the same quarter one year prior, going from $41.00 million to $47.00 million.
  • The gross profit margin for TC PIPELINES LP is currently very high, coming in at 77.01%. It has increased from the same quarter the previous year. Along with this, the net profit margin of 54.02% significantly outperformed against the industry average.
  • Investors have apparently begun to recognize positive factors similar to those we have mentioned in this report, including earnings growth. This has helped drive up the company's shares by a sharp 29.28% over the past year, a rise that has exceeded that of the S&P 500 Index. We feel that the stock's sharp appreciation over the last year has driven it to a price level which is now somewhat expensive compared to the rest of its industry. The other strengths this company shows, however, justify the higher price levels.
  • Despite the weak revenue results, TCP has outperformed against the industry average of 19.6%. Since the same quarter one year prior, revenues slightly dropped by 1.1%. The declining revenue has not hurt the company's bottom line, with increasing earnings per share.
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INT ChartINT data by YCharts
2. World Fuel Services Corporation (INT - Get Report)

Rating: Buy, A-
Market Cap: $4.05 billion
Year-to-date return: 19.6%

World Fuel Services Corporation, a fuel logistics, transaction management, and payment processing company, sells and distributes fuel and related products and services in the aviation, marine, and land transportation industries. It operates through three segments: Aviation, Marine, and Land.

"We rate WORLD FUEL SERVICES CORP (INT) a BUY. This is based on the convergence of positive investment measures, which should help this stock outperform the majority of stocks that we rate. The company's strengths can be seen in multiple areas, such as its solid stock price performance, growth in earnings per share, increase in net income, largely solid financial position with reasonable debt levels by most measures and attractive valuation levels. We feel these strengths outweigh the fact that the company has had somewhat disappointing return on equity."

Highlights from the analysis by TheStreet Ratings Team goes as follows:

  • Powered by its strong earnings growth of 28.76% and other important driving factors, this stock has surged by 29.58% over the past year, outperforming the rise in the S&P 500 Index during the same period. Regarding the stock's future course, although almost any stock can fall in a broad market decline, INT should continue to move higher despite the fact that it has already enjoyed a very nice gain in the past year.
  • WORLD FUEL SERVICES CORP has improved earnings per share by 28.8% in the most recent quarter compared to the same quarter a year ago. The company has demonstrated a pattern of positive earnings per share growth over the past two years. We feel that this trend should continue. During the past fiscal year, WORLD FUEL SERVICES CORP increased its bottom line by earning $3.11 versus $2.84 in the prior year. This year, the market expects an improvement in earnings ($3.40 versus $3.11).
  • The net income growth from the same quarter one year ago has significantly exceeded that of the S&P 500 and the Oil, Gas & Consumable Fuels industry. The net income increased by 29.4% when compared to the same quarter one year prior, rising from $51.86 million to $67.13 million.
  • The current debt-to-equity ratio, 0.37, is low and is below the industry average, implying that there has been successful management of debt levels. Along with the favorable debt-to-equity ratio, the company maintains an adequate quick ratio of 1.17, which illustrates the ability to avoid short-term cash problems.
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WNR ChartWNR data by YCharts
1. Western Refining Inc. (WNR)
Rating: Buy, A
Market Cap: $4.13 billion
Year-to-date return: 14.6%

Western Refining, Inc. operates as an independent crude oil refiner and marketer of refined products. The company operates in four segments: Refining, NTI, WNRL, and Retail.

"We rate WESTERN REFINING INC (WNR) a BUY. This is based on the convergence of positive investment measures, which should help this stock outperform the majority of stocks that we rate. The company's strengths can be seen in multiple areas, such as its revenue growth, notable return on equity, attractive valuation levels, good cash flow from operations and solid stock price performance. We feel these strengths outweigh the fact that the company has had generally high debt management risk by most measures that we evaluated."

Highlights from the analysis by TheStreet Ratings Team goes as follows:

  • The revenue growth came in higher than the industry average of 19.6%. Since the same quarter one year prior, revenues slightly increased by 0.1%. Growth in the company's revenue appears to have helped boost the earnings per share.
  • Compared to where it was a year ago today, the stock is now trading at a higher level, reflecting both the market's overall trend during that period and the fact that the company's earnings growth has been robust. Turning our attention to the future direction of the stock, it goes without saying that even the best stocks can fall in an overall down market. However, in any other environment, this stock still has good upside potential despite the fact that it has already risen in the past year.
  • The company's current return on equity greatly increased when compared to its ROE from the same quarter one year prior. This is a signal of significant strength within the corporation. Compared to other companies in the Oil, Gas & Consumable Fuels industry and the overall market, WESTERN REFINING INC's return on equity significantly exceeds that of both the industry average and the S&P 500.
  • The net income growth from the same quarter one year ago has significantly exceeded that of the S&P 500 and the Oil, Gas & Consumable Fuels industry. The net income increased by 1886.8% when compared to the same quarter one year prior, rising from -$7.33 million to $130.94 million.
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