Value stocks should arguably form the basis of any well-rounded retirement portfolio. However, it's not always easy to identify stocks that are currently trading at discounts relative to their peers that can ultimately stand the test of time.

Keeping this theme in mind, we turned to three of our contributors to ask for their thoughts on the matter. They suggested that Gilead Sciences (NASDAQ:GILD), Amgen (NASDAQ:AMGN), and General Motors (NYSE:GM) might all be undervalued stocks at the moment that can produce substantial returns over the long term -- making them perfect value stocks for a retirement portfolio. Read on to find out why.

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This top biotech is down but not out

George Budwell (Gilead Sciences): Among biotechs, Gilead is easily the most undervalued name in the space. At a price to sales ratio of 3.0 and a forward price to earnings ratio of 6.9, after all, there simply isn't a cheaper large-cap biotech stock in existence.

Truth be told, though, this rock-bottom valuation is the by-product of the company's weakening hepatitis C drug sales -- combined with the growing fear that generic HIV medicines will cut into Gilead's other major franchise, starting perhaps as early as next year.   

Putting the doom and gloom aside for the moment, Gilead is arguably still a tremendous bargain -- even for risk-adverse investors, and despite these ongoing headwinds.

The heart of the matter is that Gilead's diverse and robust clinical pipeline is being valued at pretty much nothing by the market right now, and that's bordering on the absurd based on the commercial prospects of the company's clinical portfolio.

At present, Gilead sports assets for high-value indications such as nonalcoholic steatohepatitis and rheumatoid arthritis, along with a host of anti-cancer agents in early to mid-stage development. In short, the company's pipeline could end up producing multiple blockbuster drugs over the next four to five years. 

In addition to a bearish reaction to Gilead's hep C franchise, the market has taken such a pessimistic stance toward Gilead's pipeline because of the company's setbacks in oncology. But the past is rarely prologue in the world of clinical trials -- meaning that the market's dire outlook toward Gilead's pipeline is probably wholly unjustified. 

All things considered, Gilead offers an enormous amount of under-appreciated deep value that makes it a far safer bet than the market is giving it credit for right now.  

A healthy addition to your retirement portfolio

Dan Caplinger (Amgen): Biotechnology stocks might not seem like the most obvious choice for conservative investors, but the biotech industry has come a long way in the past 20 years, and established giants like Amgen now have enough of a track record to give investors comfort in their ability to continue to grow. The stock carries a 3% dividend yield, which is well above the roughly 2% average yield for the broader stock market. Moreover, Amgen trades at 15 times its training earnings, which is well below the typical stock's earnings multiple currently.

Amgen isn't entirely without risk because, like any other company in the healthcare space, it faces challenges from competition and innovation. The key bone-marrow stimulating drug Neulasta managed to post modest sales growth year over year during the most recent quarter, but other important drugs like Epogen and Neupogen saw small declines in revenue. However, Amgen has a stable of pipeline treatments that could be tomorrow's blockbusters, and many believe the company could decide to go into the market to acquire another company to bolster its product line going forward. Even though Amgen's earnings growth isn't likely to accelerate at the pace it did when it was a younger company, investors can still feel comfortable knowing the biotech giant will take the steps it needs to take in order to foster and sustain its growth going forward.

General Motors stock is solid as a rock

Rich Smith (General Motors): Here's a value stock idea that will surprise exactly nobody: General Motors.

Let me start off by stating the obvious: At five times earnings, and only a little more than five time forward earnings, General Motors stock looks ridiculously cheap. The fact that analysts who follow GM stock expect the company to grow earnings at 10% annually over the next five years -- resulting in a PEG ratio of 0.5 -- makes the valuation even more ridiculous. And to top it all off, General Motors pays a 4.4% dividend yield, which in and of itself is probably enough to cover the cost of the stock at this low P/E -- even if there were no growth at all expected.

But, 10% growth is expected by many, and you can expect to find that growth coming from any number of directions, including the company's well-regarded Chevrolet Volt hybrid electric car, the all-electric Chevy Bolt, a new car-sharing subsidiary, and even GM's investment in ride-hailing start-up Lyft. All of this makes General Motors a value stock with a growth kicker.

As for what makes GM stock a value stock "perfect for retirement," I'd argue this: General Motors has been in business for 120 years, overcoming any number of rivals along the way. This is not a company that's going to disappear overnight. With a strong and established business selling cars and trucks today, and plenty of irons in the fire, preparing for how cars and trucks will change in the future, GM's going to be around for a long time.

I can't imagine a much more attractive price at which to buy GM stock and stick it in your retirement portfolio. 

Dan Caplinger has no position in any stocks mentioned. George Budwell has no position in any stocks mentioned. Rich Smith has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Gilead Sciences. The Motley Fool has the following options: short June 2017 $70 calls on Gilead Sciences. The Motley Fool has a disclosure policy.