Here at Zacks, our focus is on the proven Zacks Rank system, which emphasizes earnings estimates and estimate revisions to find great stocks. Nevertheless, we are always paying attention to the latest value, growth, and momentum trends to underscore strong picks.

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 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 stock to keep an eye on is Vista Oil & Gas (VIST Quick QuoteVIST - Free Report) . VIST 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 4.23, while its industry has an average P/E of 8.11. Over the last 12 months, VIST's Forward P/E has been as high as 5.57 and as low as 2.83, with a median of 3.77.

Finally, we should also recognize that VIST has a P/CF ratio of 3.68. This metric takes into account a company's operating cash flow and can be used to find stocks that are undervalued based on their solid cash outlook. VIST's P/CF compares to its industry's average P/CF of 4.82. Within the past 12 months, VIST's P/CF has been as high as 3.72 and as low as 1.63, with a median of 3.06.

Investors could also keep in mind YPF Sociedad Anonima (YPF Quick QuoteYPF - Free Report) , an Oil and Gas - Integrated - International stock with a Zacks Rank of # 1 (Strong Buy) and Value grade of A.

YPF Sociedad Anonima also has a P/B ratio of 0.45 compared to its industry's price-to-book ratio of 1.42. Over the past year, its P/B ratio has been as high as 0.48, as low as 0.11, with a median of 0.24.

Value investors will likely look at more than just these metrics, but the above data helps show that Vista Oil & Gas and YPF Sociedad Anonima are likely undervalued currently. And when considering the strength of its earnings outlook, VIST and YPF sticks out as one of the market's strongest value stocks.