Big Pharma has been going through a number of major changes lately due to the so-called patent cliff and the advent of a host of game-changing new drugs. As a result, shares of some top pharmas have taken flight, while others have muddled along following loss of patent exclusivity.

Source: Wikimedia.

Going forward, more upheaval is anticipated in the pharmaceutical investing landscape even as payers push back against costly new medicines, and biosimilars -- the generic equivalents to drugs developed from biological sources -- take hold in the U.S.  

Given all the headwinds facing the industry, we asked three of our top healthcare analysts to name a Big Pharma stock they believe is actually worth investing in right now. Here's what they had to say.

George Budwell: I think Merck & Co. (NYSE:MRK) will be one of the best-performing Big Pharma stocks over the next decade, making it worth a look by patient investors.

Unpacking this assertion a bit, I'm encouraged by Merck's better-than-average dividend yield and its long track record of rewarding shareholders with frequent share repurchases. In the past 12 months, for example, the company spent $13 billion on share buybacks and dividends; in addition, the present yield of 3.1% easily tops the sector average of 2.2%.

What might scare off some investors, though, is the marked drop in Merck's pharmaceutical revenue since 2012.

Given the loss of exclusivity for key products including asthma drug Singulair (the drug's sales fell from $3.9 billion in 2012 to $1.1 billion in 2014) and a strong dollar negatively impacting sales numbers, total pharma sales have declined by 4.3%, on average, over the last two years. With the blockbuster anti-inflammatory drug Remicade coming off patent protection in the European Union, this trend might continue for the foreseeable future.

The good news is that Merck has been slashing costs and developing its next generation of blockbuster candidates, including the cancer-fighting PD-1 inhibitor Keytruda.

Last year, Keytruda received an accelerated approval for advanced melanoma in the U.S., where it raked in $50 million in fourth-quarter sales. With the drug now under regulatory review as a treatment for non-small cell lung cancer, it has real potential to help turn Merck's pharma sales around in a hurry. In fact, the drug's annual sales are estimated to hover around $3 billion to $4 billion by as early as 2020, with more optimistic projections even reaching into the $6 billion range. 

On a final note, Merck is also one of the few Big Pharma stocks that actually looks undervalued, in terms of future prospects. Because of its aggressive share repurchase program, multibillion-dollar cost-cutting initiatives, and the strong launch for Keytruda, Merck is expected to grow earnings this year by a whopping 176%. If this line holds true, the company's shares are trading at a forward price-to-earnings ratio of only 15, well below many of its peers.

Todd Campbell:  It's been tough sledding for Pfizer (NYSE:PFE) since losing patent protection on its megablockbuster cholesterol drug Lipitor, but better times might be coming. If so, investors may want to stash this Big Pharma away in their portfolios.

Lipitor was racking up sales of more than $12 billion per year before its patent expired in 2011, and that top-line hit hasn't been easily offset. Now that Lipitor's slide has slowed, new drugs and a big acquisition could mean Pfizer's revenue will climb again.

That growth could come in part from breast cancer drug Ibrance. Ibrance won FDA approval earlier this year for use in breast cancer patients with a specific genetic makeup, and analysts think it could be a $3 billion a year drug someday. Pfizer is also developing potential blockbuster RN316, a cholesterol-busting drug that targets the PCSK9 protein.

Those drugs are intriguing, but biosimilars could be an even bigger driver of future growth. Biosimilars are generic alternatives to hard-to-copy biologic drugs, and the FDA approved the first one this year. Since healthcare payers are likely to drive demand for biosimilars over time, Pfizer is acquiring biosimilars leader Hospira. That acquisition could produce billions of dollars in additional revenue in coming years that could help Pfizer become a growth stock again.

Cheryl Swanson: With its recent acquisition of Allergan, generic-drug maker Actavis (NYSE:AGN) is poised to become the world's 10th-largest drug company. While the company provides no dividend, and Merck and Pfizer are both great picks, Actavis is an interesting candidate for more aggressive, growth-oriented investors.  

It says something about Wall Street's ruthless focus on mergers and acquisitions that Actavis is worth almost twice as much as it was a year ago. In that time frame, the share price is up 44%, far exceeding the performance of the broader market. But there's plenty of room for more growth, as the company's earnings have kept pace. In fact, shares are attractively valued at a forward earnings multiple of less than 14. 

CEO Brent Saunders has a great history of integrating companies, creating synergies, and achieving growth targets. His track record makes it likely the company will achieve its target of $1.8 billion in synergies with the Allergan acquisition. Saunders has also promised to keep Allergan's research and development budget intact, allowing it to continue crucial research on dry-eye drugs and the continued rapid label expansion for Botox.

On the downside, Actavis has taken on substantial debt with its acquisition spree. But it has also exceeded expectations for debt reduction by using strong cash flow to pay down obligations. In fact, Actavis' current total cash flow growth is over 100%, a level far higher than the industry average. I've owned Actavis for quite a while, but with the company in a pullback, I may up my position. While integration issues will likely make Actavis' performance volatile this year, for those willing to accept higher risk, it's a stock well worth considering.