While the proven Zacks Rank places an emphasis on earnings estimates and estimate revisions to find strong stocks, we also know that investors tend to develop their own individual strategies. With this in mind, we are always looking at value, growth, and momentum trends to discover great companies.Of these, perhaps no stock market trend is more popular than value investing, which is a strategy that has proven to be successful in all sorts of market environments. Value investors rely on traditional forms of analysis on key valuation metrics to find stocks that they believe are undervalued, leaving room for profits.Luckily, Zacks has developed its own Style Scores system in an effort to find stocks with specific traits. Value investors will be interested in the system's "Value" category. Stocks with both "A" grades in the Value category and high Zacks Ranks are among the strongest value stocks on the market right now.Aflac (AFL) is a stock many investors are watching right now. AFL is currently sporting a Zacks Rank of #2 (Buy), as well as an A grade for Value. The stock holds a P/E ratio of 10.09, while its industry has an average P/E of 10.29. Over the past year, AFL's Forward P/E has been as high as 11.36 and as low as 7.08, with a median of 9.34.Investors should also note that AFL holds a PEG ratio of 2.02. 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. AFL's industry currently sports an average PEG of 2.04. Over the past 52 weeks, AFL's PEG has been as high as 2.52 and as low as 1.42, with a median of 1.89.Value investors also frequently use the P/S ratio. This metric is found by dividing a stock's price with the company's revenue. This is a prefered metric because revenue can't really be manipulated, so sales are often a truer performance indicator. AFL has a P/S ratio of 1.59. This compares to its industry's average P/S of 1.62.Value investors will likely look at more than just these metrics, but the above data helps show that Aflac is likely undervalued currently. And when considering the strength of its earnings outlook, AFL sticks out at as one of the market's strongest value stocks. Breakout Biotech Stocks with Triple-Digit Profit Potential The biotech sector is projected to surge beyond $775 billion by 2024 as scientists develop treatments for thousands of diseases. They’re also finding ways to edit the human genome to literally erase our vulnerability to these diseases. Zacks has just released Century of Biology: 7 Biotech Stocks to Buy Right Now to help investors profit from 7 stocks poised for outperformance. Our recent biotech recommendations have produced gains of +50%, +83% and +164% in as little as 2 months. The stocks in this report could perform even better.See these 7 breakthrough stocks now>>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Aflac Incorporated (AFL): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research