Sovaldi pill. Source: Gilead Sciences

Gilead Sciences (NASDAQ:GILD) had another phenomenal year in 2014, returning 26% for investors. That's more than double the return of the broad-based S&P 500, and it has hepatitis C drug Sovaldi to thank. 

In just the first nine months of 2014, Sovaldi netted Gilead $8.55 billion in sales. It's the most effective drug launch in history by a mile, and potentially the second best-selling drug in the world in 2014.

But, have Gilead's best days come and gone? We asked three of our top analysts to name one biotech stock they believe could outperform Gilead Sciences in 2015, and here's what they had to say.

Todd Campbell: There are plenty of reasons to look beyond AbbVie's current threat to Gilead Sciences, but for investors who would rather avoid Gilead Sciences altogether, Celgene Corp (NASDAQ:CELG) is one worth considering.

Celgene is one of biotech's shining stars. It has two top selling multiple myeloma drugs, including Revlimid, which is the leading second-line therapy, and Pomalyst, a fast-growing third line therapy.

Celgene expects that Revlimid sales, which are currently running at around $5 billion per year, will eclipse $7 billion by 2017. Pomalyst, which saw its sales double to $181 million in the third quarter, could be a billion-dollar plus drug by 2017, too. Additionally, Celgene markets the cancer drug Abraxane, which posted sales of $212 million last quarter, and the company launched its first autoimmune disease drug -- the psoriasis pill Otezla -- last fall.

Source: Celgene.

In addition to having a stable of top sellers, Celgene is one of the most committed companies in terms of R&D spending. Celgene has spent 35% of its revenue on R&D over the past 12 months, while Gilead Sciences and Amgen have spent just 22% and 11%, respectively. Celgene's commitment to developing next generation drugs includes both in-house therapies and partnerships with some of the most intriguing emerging biotechs out there, including Agios and bluebird bio. That focus on the future should mean that investors see a steady stream of new needle-moving therapies over the coming decade. If so, then shareholders should be well rewarded for taking a stake in the company now.

Sean Williams: Gilead Sciences had an impressive 2014, and it's certainly a deeply discounted value play heading into the new year, but I'm going to pull the "homer" pick out of the bag and select small-cap Exelixis (NASDAQ:EXEL), a personal portfolio holding of mine, as a company that could have a remarkably better year.

Source: Roche.

Understandably, Exelixis isn't without risk. The company's lone Food and Drug Administration approved drug, Cometriq, didn't extend overall survival in a statistically significant manner in the phase 3 COMET-1 study on metastatic castration-resistant prostate cancer, or mCRPC. Furthermore, Exelixis isn't going to be profitable anytime soon, meaning it could turn to dilutive share offerings to raise cash.

However, my excitement stems from the expected release of phase 3 METEOR data in the second quarter. METEOR is a late-stage study looking at Cometriq as a treatment for advanced renal cell carcinoma. Unlike the mCRPC trial, the primary endpoint of this study won't be overall survival. Instead, it's progression-free survival, an area in which Cometriq did hit statistical significance in the COMET-1 study. While mCRPC was a more lucrative label, having renal cell carcinoma in its pocket, along with its existing metastatic medullary thyroid cancer indication, could get Exelixis near profitability.

Also, the coBRIM study involving Roche's Zelboraf and Exelixis-developed cobimetinib delivered excellent late-stage data as treatment for BRAF V600 mutation-positive advanced melanoma and has the potential to be approved in the U.S. by late 2015. If the cards align just right, Exelixis could have an exceptionally good year. But, then again, I do own Exelixis, so consider where the bias is coming from.

Brian Orelli: I like Sean's pick; I actually picked Exelixis as my stock that could double next year. But if our goal is just to beat Gilead Sciences, I'm not sure we need to go with something quite that binary.

Source: Biogen Idec.

Instead, I'll stick in the large-cap biotech realm and pick Biogen Idec (NASDAQ:BIIB). Its oral multiple sclerosis drug, Tecfidera, is still ramping up sales quickly, especially outside of the U.S. And a pair of newly-approved drugs for hemophilia, Alprolix and Eloctate, are off to a good start. The drugs won't become blockbusters as quickly as Tecfidera, but they should offer steady growth.

Biogen Idec is big enough that its revenue and earnings growth will drive most of the stock movement, but the biotech does have a big a binary event coming up in 2015 that may need to come out positive if it's going to get over the beating-Gilead hump. Namely, this pivotal moment will be the announcement of data from two phase 2 clinical trials testing its anti-LINGO-1 antibody.

LINGO-1 promotes the degradation of the protective myelin sheath around nerve fibers. Those exposed nerve fibers cause the underlying problem in many neuro-degenerative diseases. Biogen's antibody drug seeks to block the protein, potentially allowing the body to counteract the disease and regrow the myelin sheath.

A proof-of-concept trial in optic neuritis will read out in January, and another in multiple sclerosis -- a larger potential market -- will read out later in the year. If the trials are successful, the potential future sales will be added to long-term growth estimates, which should boost the stock price.