NEW YORK (TheStreet) - Although many airlines are having sky-high profits this year, some are experiencing turbulence. Delta Air Lines  (DAL - Get Report) and Southwest Airlines (LUV - Get Report) have had their employees reject proposed labor contracts due to low pay in light of the airlines' recent success.

The first half of this year, U.S. airline earnings were north of $8 billion, collectively. Low oil prices have boosted airline profits -- not only because it's at a lower cost for them, but also because more people are traveling due to more disposable income.

Although Delta Air Lines and Southwest Airlines did not make our list of rock-solid airline stocks to buy, they were both "buys" according to TheStreet Quant Ratings, with "A-" letter grades.

So, what are the best airlines investors should be buying? Here are the top four, 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 stocks made the list. And when you're done, be sure to read about which pharmaceutical stocks you should buy now. Year-to-date returns are based on August 19, 2015 closing prices as of 10:27AM. The highest-rated stock appears last.

ALGT ChartALGT data by YCharts
4. Allegiant Travel Company (ALGT - Get Report)

Rating: Buy, A
Market Cap: $3.9 billion
Year-to-date return: 50.2%

Allegiant Travel Company, a leisure travel company, focuses on the provision of travel services and products to residents of under-served cities in the United States.

"We rate ALLEGIANT TRAVEL CO (ALGT) 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, expanding profit margins, good cash flow from operations and compelling growth in net income. We feel its strengths outweigh the fact that the company is trading at a premium valuation based on our review of its current price compared to such things as earnings and book value."

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

  • The revenue growth came in higher than the industry average of 7.4%. Since the same quarter one year prior, revenues rose by 10.9%. Growth in the company's revenue appears to have helped boost the earnings per share.
  • 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. When compared to other companies in the Airlines industry and the overall market, ALLEGIANT TRAVEL CO's return on equity exceeds that of the industry average and significantly exceeds that of the S&P 500.
  • 43.63% is the gross profit margin for ALLEGIANT TRAVEL CO which we consider to be strong. It has increased from the same quarter the previous year. Along with this, the net profit margin of 16.86% is above that of the industry average.
  • Net operating cash flow has significantly increased by 70.13% to $67.79 million when compared to the same quarter last year. In addition, ALLEGIANT TRAVEL CO has also vastly surpassed the industry average cash flow growth rate of 14.43%.
  • The company, on the basis of net income growth from the same quarter one year ago, has significantly outperformed against the S&P 500 and exceeded that of the Airlines industry average. The net income increased by 62.2% when compared to the same quarter one year prior, rising from $33.50 million to $54.34 million.

JBLU ChartJBLU data by YCharts
3. JetBlue Airways Corporation (JBLU - Get Report)

Rating: Buy, A
Market Cap: $7.6 billion
Year-to-date return: 51.6%

JetBlue Airways Corporation, a passenger carrier company, provides air transportation services. As of December 31, 2014, the company operated a fleet of 13 Airbus A321 aircrafts, 130 Airbus A320 aircrafts, and 60 EMBRAER 190 aircrafts.

"We rate JETBLUE AIRWAYS CORP (JBLU) 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, good cash flow from operations, expanding profit margins, largely solid financial position with reasonable debt levels by most measures and notable return on equity. We feel its strengths outweigh the fact that the company has had sub par growth in net income."

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

  • The revenue growth came in higher than the industry average of 7.4%. Since the same quarter one year prior, revenues slightly increased by 8.0%. This growth in revenue does not appear to have trickled down to the company's bottom line, displayed by a decline in earnings per share.
  • Net operating cash flow has significantly increased by 65.29% to $362.00 million when compared to the same quarter last year. In addition, JETBLUE AIRWAYS CORP has also vastly surpassed the industry average cash flow growth rate of 14.43%.
  • 37.78% is the gross profit margin for JETBLUE AIRWAYS CORP which we consider to be strong. It has increased from the same quarter the previous year. Regardless of the strong results of the gross profit margin, the net profit margin of 9.42% trails the industry average.
  • The debt-to-equity ratio is somewhat low, currently at 0.73, and is less than that of the industry average, implying that there has been a relatively successful effort in the management of debt levels. Even though the company has a strong debt-to-equity ratio, the quick ratio of 0.48 is very weak and demonstrates a lack of ability to pay short-term obligations.
  • The return on equity has improved slightly when compared to the same quarter one year prior. This can be construed as a modest strength in the organization. Compared to other companies in the Airlines industry and the overall market on the basis of return on equity, JETBLUE AIRWAYS CORP has underperformed in comparison with the industry average, but has exceeded that of the S&P 500.

ALK ChartALK data by YCharts
2. Alaska Air Group, Inc. (ALK - Get Report)

Rating: Buy, A+
Market Cap: $10.3 billion
Year-to-date return: 35.8%

Alaska Air Group, Inc., through its subsidiaries, provides passengers and cargo air transportation services primarily in the United States. The company operates through Alaska Mainline and Alaska Regional segments.

"We rate ALASKA AIR GROUP INC (ALK) 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, largely solid financial position with reasonable debt levels by most measures, solid stock price performance, impressive record of earnings per share growth and compelling growth in net income. Although the company may harbor some minor weaknesses, we feel they are unlikely to have a significant impact on results."

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

  • The revenue growth came in higher than the industry average of 7.4%. Since the same quarter one year prior, revenues slightly increased by 4.5%. Growth in the company's revenue appears to have helped boost the earnings per share.
  • The current debt-to-equity ratio, 0.34, is low and is below the industry average, implying that there has been successful management of debt levels.
  • Powered by its strong earnings growth of 50.42% and other important driving factors, this stock has surged by 77.50% 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, ALK should continue to move higher despite the fact that it has already enjoyed a very nice gain in the past year.
  • ALASKA AIR GROUP INC reported significant earnings per share improvement 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, ALASKA AIR GROUP INC increased its bottom line by earning $4.43 versus $3.59 in the prior year. This year, the market expects an improvement in earnings ($6.30 versus $4.43).
  • The net income growth from the same quarter one year ago has greatly exceeded that of the S&P 500, but is less than that of the Airlines industry average. The net income increased by 41.8% when compared to the same quarter one year prior, rising from $165.00 million to $234.00 million.

RYAAY ChartRYAAY data by YCharts
1. Ryanair Holdings plc (RYAAY - Get Report)

Rating: Buy, A+
Market Cap: $20.4 billion
Year-to-date return: 5%

Ryanair Holdings plc, together with its subsidiaries, provides scheduled-passenger airline services in Ireland, the United Kingdom, continental Europe, and Morocco.

"We rate RYANAIR HOLDINGS PLC (RYAAY) 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, solid stock price performance, notable return on equity, largely solid financial position with reasonable debt levels by most measures and increase in net income. We feel its strengths outweigh the fact that the company shows weak operating cash flow."

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

  • RYANAIR HOLDINGS PLC's earnings per share improvement from the most recent quarter was slightly positive. 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, RYANAIR HOLDINGS PLC increased its bottom line by earning $3.35 versus $2.50 in the prior year. This year, the market expects an improvement in earnings ($3.97 versus $3.35).
  • The return on equity has improved slightly when compared to the same quarter one year prior. This can be construed as a modest strength in the organization. Compared to other companies in the Airlines industry and the overall market on the basis of return on equity, RYANAIR HOLDINGS PLC has underperformed in comparison with the industry average, but has exceeded that of the S&P 500.
  • 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 43.42% 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.
  • RYAAY, with its decline in revenue, slightly underperformed the industry average of 7.4%. Since the same quarter one year prior, revenues slightly dropped by 10.0%. The declining revenue has not hurt the company's bottom line, with increasing earnings per share.
  • Even though the current debt-to-equity ratio is 1.08, it is still below the industry average, suggesting that this level of debt is acceptable within the Airlines industry. Regardless of the somewhat mixed results with the debt-to-equity ratio, the company's quick ratio of 1.45 is sturdy.