Investors interested in stocks from the Containers - Paper and Packaging sector have probably already heard of Greif (GEF) and AptarGroup (ATR). But which of these two companies is the best option for those looking for undervalued stocks? Let's take a closer look.We have found that the best way to discover great value opportunities is to pair a strong Zacks Rank with a great grade in the Value category of our Style Scores system. The proven Zacks Rank emphasizes companies with positive estimate revision trends, and our Style Scores highlight stocks with specific traits.Greif has a Zacks Rank of #1 (Strong Buy), while AptarGroup has a Zacks Rank of #4 (Sell) right now. The Zacks Rank favors stocks that have recently seen positive revisions to their earnings estimates, so investors should rest assured that GEF has an improving earnings outlook. But this is only part of the picture 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.Our Value category highlights undervalued companies by looking at a variety of key metrics, including the popular P/E ratio, as well as the P/S ratio, earnings yield, cash flow per share, and a variety of other fundamentals that have been used by value investors for years.GEF currently has a forward P/E ratio of 12.93, while ATR has a forward P/E of 31.50. We also note that GEF has a PEG ratio of 1.29. This popular figure is similar to the widely-used P/E ratio, but the PEG ratio also considers a company's expected EPS growth rate. ATR currently has a PEG ratio of 4.50.Another notable valuation metric for GEF is its P/B ratio of 2.19. The P/B ratio is used to compare a stock's market value with its book value, which is defined as total assets minus total liabilities. For comparison, ATR has a P/B of 4.13.These are just a few of the metrics contributing to GEF's Value grade of B and ATR's Value grade of C.GEF is currently sporting an improving earnings outlook, which makes it stick out in our Zacks Rank model. And, based on the above valuation metrics, we feel that GEF is likely the superior value option right now. Tech IPOs With Massive Profit Potential In the past few years, many popular platforms and like Uber and Airbnb finally made their way to the public markets. But the biggest paydays came from lesser-known names. For example, electric carmaker X Peng shot up +299.4% in just 2 months. Think of it this way… If you had put $5,000 into XPEV at its IPO in September 2020, you could have cashed out with $19,970 in November. With record amounts of cash flooding into IPOs and a record-setting stock market, this year’s lineup could be even more lucrative.See Zacks Hottest Tech IPOs 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 Greif, Inc. (GEF): Free Stock Analysis Report AptarGroup, Inc. (ATR): Free Stock Analysis Report To read this article on Zacks.com click here.