Source: Bill Ackman via Insider Monkey

Investors owning healthcare stocks may be sad to see 2014 come to an end. The S&P healthcare index performance more than doubled the S&P 500 return, and biotechnology stocks have been among the market's top performers this year. No one knows what's in store for healthcare stocks in 2015, but before we bid 2014 adieu, we asked three Motley Fool analysts to tell us what highlight (or lowlight) is most memorable to them. Read on to see what they think were health care's biggest stories this year.

Todd Campbell: So many choices! What a year it's been for healthcare and biotechnology in particular, but if I have to pick one most memorable event it has to be the rise of the investor activist. These activists have always existed, but until now most of their efforts have been behind-the-scenes.

Activist Bill Ackman, the portfolio manager behind the hedge fund Pershing Square Capital, made one of the biggest splashes in April when he got together with Valeant Pharmaceuticals (NYSE:BHC) in an attempt to coordinate the acquisition of Botox and ophthalmology powerhouse Allergan (NYSE: AGN).

Recognizing that big money could be shaved from Allergan's expenses, Ackman took a nearly 10% ownership stake in Allergan, and Valeant held up its end by making a $47 billion buyout bid for the company. Despite that offer being well above Allergan's $37 billion market cap in March, Allergan rebuffed the deal, kicking off a war-of-words, special meetings, and court proceedings. The back-and-forth between Ackman and Allergan's board continued until November, when generic drugmaker Actavis (NYSE:AGN) trumped Valeant with a bid worth a whopping $66 billion. Actavis' offer proves that Allergan's board was right, but no one will shed tears for Ackman. After all, Actavis' higher bid means that when all is said and done he'll have made more than $2 billion for his investors. Whether Ackman and activists hold as much sway again next year isn't certain, but given his recent stake in Zoetis investors are likely to keep watching him.

George Budwell: When it comes to memorable moments in healthcare this year, I have to go with the battle between the Food and Drug Administration and Sarepta Therapeutics (NASDAQ:SRPT) over the company's Duchenne muscular dystrophy therapy eteplirsen. After the FDA declined to review eteplirsen in November 2013, the agency seemingly did an about face this year, allowing Sarepta to proceed with a regulatory filing, with the provision that a late-stage trial would be initiated prior to the drug's conditional approval.

Last October, though, the FDA reversed course again. In a public statement, the agency said that they now have serious concerns over data taken at a particular clinical trial site, and will therefore require independent verification of the 168-week clinical data, MRI data, and the patient-level natural history data. Adding insult to injury, the FDA is also requiring further safety studies prior to even a conditional approval.

Why is this back and forth memorable? In my mind, Sarepta's roller coaster-like dealings with the FDA are a prime example of just how much uncertainty surrounds the development of any novel therapy. That's why I tend to avoid buying clinical-stage biopharmas in general.

Leo Sun: Pfizer's (NYSE:PFE) failed attempt to acquire AstraZeneca (NASDAQ:AZN) for $118 billion highlighted two key issues for Big Pharma companies -- tax inversions and drying pipelines.

By acquiring AstraZeneca, Pfizer could have moved its tax address from New York to London, reducing its corporate tax rate from around 27.4% to 21.3% in a practice known as tax inversion. Barclays estimated that a single percentage point in tax reduction could add $200 million to Pfizer's net income, resulting in annual savings of over $1.2 billion. A recent flood of similar tax inversions by U.S. companies resulted in new tax laws, which required at least 40% (up from 20%) of a tax inverting company to be owned by foreign shareholders.

Pfizer also wanted AstraZeneca's promising pipeline of diabetes and cancer drugs to help replace its former blockbuster cholesterol drug Lipitor, which lost patent protection in 2011. Several of Pfizer's late-stage drug candidates -- like palbociclib and bococizumab -- look promising, but they'll come nowhere close to matching the $11 billion in peak sales Lipitor once generated. Meanwhile, AstraZeneca forecast that its promising pipeline of drugs could lift its annual sales 75% to $45 billion by 2023.

The deal ultimately fell apart because AstraZeneca believed that Pfizer undervalued the company, but the saga highlights how hard it can be for massive pharmaceutical companies to merge and grow inorganically. 

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.