Will these underdogs come out on top? Image source: Getty Images.

Whether they're companies that are challenging the giants of their industry or former high-flyers that have lost their luster, underdog stocks can make -- or break -- a portfolio. We asked three of our top contributors each to discuss a stock that fits the "underdog" category; they gave us a well-known consumer brand that has lost some momentum, a biotech that's working on a blockbuster, and a recently merged alternative-fuels innovator that's trying to move from development to commercial success. 

While now might not be the right time to buy shares of these underdogs, our contributors are watching them closely. Here's what they had to say about GoPro Inc. (NASDAQ:GPRO)Omeros Corp. (NASDAQ:OMER), and Westport Fuel Systems Inc. (USA) (NASDAQ:WPRT)

Steve Symington: As far as "underdogs" go in today's market, arguably no company better fits the bill than GoPro. Shares of the action-camera maker are down more than 80% over the past year, thanks to a string of disappointing earnings reports as demand for its flagship capture devices has waned.

Last month, GoPro told investors that first-quarter 2016 revenue declined a harrowing 49.5% year over year, to $183.5 million. Meanwhile, GoPro's adjusted earnings before interest, taxes, depreciation, and amortization (EBITDA) came in at a loss of $86.8 million, and its adjusted net loss was $86.7 million, or $0.63 per share. To make matters worse, GoPro pushed back the expected first-half 2016 launch of its Karma quadcopter to the second half of the year (but still in time for the holidays), citing the need to further fine-tune what it described as "revolutionary features that differentiated it from other drones."

However, it's worth noting these seemingly disastrous figures were significantly better than GoPro's guidance, which called for an even steeper drop in sales to a range of $160 million to $180 million, and for an adjusted EBITDA loss of $96 million, plus or minus $2.5 million. And during the subsequent conference call, GoPro management indicated the "vast majority" of revenue it expected from the Karma drone was being modeled in the second half of the year, anyway. 

When we combine that impending launch with GoPro's relative outperformance in the first quarter, as well as a widely expected refresh of its HERO4 camera line before the holiday season, I think it's quite possible we see investor sentiment swinging squarely back in GoPro's favor before the year comes to a close.

Brian Feroldi: Omeros is an underdog stock that I've followed for some time with great interest: It stands a real chance of wreaking havoc on the business model of Alexion Pharmaceuticals (NASDAQ:ALXN), a biotech giant that I'm quite fond of. 

Right now Alexion Pharmaceuticals' best selling drug is Soliris, which is approved to treat two ultrarare diseases: paroxysmal nocturnal hemoglobinuria, or PNH, and atypical hemolytic uremic syndrome, or aHUS. Soliris is currently the only drug on the market that treats either of these diseases, and since they are both orphan indications, Alexion yields enormous pricing power. Soliris' list price tops $537,000 per year in the U.S., and worldwide sales of the drug are expected to top $2.8 billion in 2016.

A market that massive is bound to create competition, which is where Omeros comes into the picture. Omeros' most exciting product candidate is OMS721, which the company is studying as a potential treatment for aHUS. The drug has already performed well in early clinical trials, and right now it is starting Phase 3. While it's hard to handicap its chances of regulatory success, OMS721 already has both orphan-drug designation and fast-track status. To me, that hints that it has a better-than-average chance of finding its way to market.

That fact should concern Alexion Pharmaceuticals investors, since Omeros could start a price war if OMS721 does win approval. If that happens, it would greatly hinder Alexion's growth prospects, which would certainly do bad things to its lofty stock price.

It's worth pointing out that Alexion is aware of the potential threat, and it's studying Soliris for uses beyond PHN and aHUS. If those go well, they could help to soften the blow if OMS721 does eventually find its way to market.

Still, Omeros stands a chance of being a real thorn in Alexion's side if things go according to plan, which is why it is one underdog stock that I plan on watching like a hawk.

Jason Hall: Westport Fuel Systems, the recently merged combination of Westport Innovations and Fuel Systems Solutions, should make for a better company with a stronger balance sheet, better operating cash flow, and the ability to streamline operations and cut redundant costs. But at the same time, there will be a lot of challenges as the company forges ahead with the commercial launch of its HPDI 2.0 technology, targeted at driving conversion to natural gas in the heavy-duty trucking industry. 

Westport is without a doubt an underdog, though the company's approach today is to partner with industry leaders such as CumminsDaimler AG, and AB Volvo (to name a few), supplying them with the components to build natural-gas-powered engines. This shift in trajectory, in partnership with Delphi Automotive for HPDI 2.0, could be a real home run for the company in the next year or so. 

At the same time, the company must integrate two separate sets of operations, figure out its product-development priorities, and continue cutting costs to balance the books -- because all the upside in the world is useless if the company won't have enough cash flow to reach the finish line. The recent merger vastly improves the prospects for former Westport shareholders, and should mean a bigger opportunity for Fuel Systems investors. 

All that's left? The execution. I'm not buying more shares of Westport anytime soon, but I'm closely watching the integration, and looking for signs of how HPDI 2.0 will be received by the market.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.