Here at Zacks, we focus on our proven ranking system, which places an emphasis on earnings estimates and estimate revisions, to find winning stocks. But we also understand that investors develop their own strategies, so we are constantly looking at the latest trends in value, growth, and momentum to find strong companies for our readers.

Of these, value investing is easily one of the most popular ways to find great stocks in any market environment. Value investors use tried-and-true metrics and fundamental analysis to find companies that they believe are undervalued at their current share price levels.

Zacks has developed the innovative Style Scores system to highlight stocks with specific traits. For example, value investors will be interested in stocks with great grades in the "Value" category. When paired with a high Zacks Rank, "A" grades in the Value category are among the strongest value stocks on the market today.

One company value investors might notice is Alaska Air Group (ALK Quick QuoteALK - Free Report) . ALK is currently holding a Zacks Rank of #2 (Buy) and a Value grade of A. The stock is trading with a P/E ratio of 12.03, which compares to its industry's average of 16.51. Over the past 52 weeks, ALK's Forward P/E has been as high as 14.88 and as low as 6.38, with a median of 8.20.

Investors should also note that ALK holds a PEG ratio of 0.66. This metric is used similarly to the famous P/E ratio, but the PEG ratio also takes into account the stock's expected earnings growth rate. ALK's industry currently sports an average PEG of 0.83. Over the last 12 months, ALK's PEG has been as high as 1.17 and as low as 0.42, with a median of 0.59.

We should also highlight that ALK has a P/B ratio of 2.19. The P/B is a method of comparing a stock's market value to its book value, which is defined as total assets minus total liabilities. ALK's current P/B looks attractive when compared to its industry's average P/B of 5.13. ALK's P/B has been as high as 2.20 and as low as 0.99, with a median of 1.31, over the past year.

Finally, investors will want to recognize that ALK has a P/CF ratio of 10.69. This data point considers a firm's operating cash flow and is frequently used to find companies that are undervalued when considering their solid cash outlook. ALK's current P/CF looks attractive when compared to its industry's average P/CF of 11.96. Over the past year, ALK's P/CF has been as high as 10.77 and as low as 5.92, with a median of 7.35.

Delta Air Lines (DAL Quick QuoteDAL - Free Report) may be another strong Transportation - Airline stock to add to your shortlist. DAL is a # 2 (Buy) stock with a Value grade of A.

Delta Air Lines is trading at a forward earnings multiple of 8.39 at the moment, with a PEG ratio of 0.75. This compares to its industry's average P/E of 16.51 and average PEG ratio of 0.83.

Over the last 12 months, DAL's P/E has been as high as 9.94, as low as 5.44, with a median of 7.12, and its PEG ratio has been as high as 1.15, as low as 0.62, with a median of 0.78.

Furthermore, Delta Air Lines holds a P/B ratio of 2.71 and its industry's price-to-book ratio is 5.13. DAL's P/B has been as high as 3.11, as low as 1.94, with a median of 2.70 over the past 12 months.

These are just a handful of the figures considered in Alaska Air Group and Delta Air Lines's great Value grade. Still, they help show that the stock is likely being undervalued at the moment. Add this to the strength of its earnings outlook, and we can clearly see that ALK and DAL is an impressive value stock right now.