Investors interested in Oil and Gas - Integrated - International stocks are likely familiar with Shell Oil (RDS.A) and Chevron (CVX). But which of these two companies is the best option for those looking for undervalued stocks? Let's take a closer look.There are plenty of strategies for discovering value stocks, but we have found that pairing a strong Zacks Rank with an impressive grade in the Value category of our Style Scores system produces the best returns. The proven Zacks Rank emphasizes companies with positive estimate revision trends, and our Style Scores highlight stocks with specific traits.Currently, Shell Oil has a Zacks Rank of #1 (Strong Buy), while Chevron has a Zacks Rank of #3 (Hold). This system places an emphasis on companies that have seen positive earnings estimate revisions, so investors should feel comfortable knowing that RDS.A is likely seeing its earnings outlook improve to a greater extent. But this is just one piece of the puzzle for value investors.Value investors are also interested in a number of tried-and-true valuation metrics that help show when a company is undervalued at its current share price levels.The Value category of the Style Scores system identifies undervalued companies by looking at a number of key metrics. These include the long-favored P/E ratio, P/S ratio, earnings yield, cash flow per share, and a variety of other fundamentals that help us determine a company's fair value.RDS.A currently has a forward P/E ratio of 9.19, while CVX has a forward P/E of 19.74. We also note that RDS.A has a PEG ratio of 2.30. This figure is similar to the commonly-used P/E ratio, with the PEG ratio also factoring in a company's expected earnings growth rate. CVX currently has a PEG ratio of 3.95.Another notable valuation metric for RDS.A is its P/B ratio of 0.95. The P/B ratio pits a stock's market value against its book value, which is defined as total assets minus total liabilities. For comparison, CVX has a P/B of 1.44.These are just a few of the metrics contributing to RDS.A's Value grade of B and CVX's Value grade of C.RDS.A sticks out from CVX in both our Zacks Rank and Style Scores models, so value investors will likely feel that RDS.A is the better option right 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 Royal Dutch Shell PLC (RDS.A): Free Stock Analysis Report Chevron Corporation (CVX): Free Stock Analysis Report To read this article on Zacks.com click here.