Source: Flickr user stockmonkeys.com.

Biotech stocks have been some of the market's best performers, but recently, biotech stocks have been getting the cold shoulder from investors and that raises an important question: are this year's top performing biotech stocks still buys? Let's consider the three best performing big cap biotech stocks and find out if they remain worth picking up in portfolios.

No. 1: BioMarin Pharmaceuticals (BMRN -0.93%) -- up 52.2%
Sky-high prices and limited competition have made biotech stocks that target rare diseases, including BioMarin, all the rage, but BioMarin is far from a risk-free bet.

The company's got a growing stable of drugs with six-figure price tags targeting uncommon conditions, such as those caused by genetic mutations, but profit remains elusive and that means its $21.4 billion market cap is arguably lofty.

Because BioMarin's valuation may be stretched, investors considering BioMarin for portfolios need to think long term and, on that basis, a $20 billion plus market cap may not be as out of whack as it seems.

The company's second-quarter sales soared 30.6% higher to $250.5 million and that allowed it to post a net loss per share of $0. 51, which was better than the $0.53 loss that had been expected.

If sales continue growing and new therapies, including its promising treatment for Duchenne's Muscular Dystrophy that is under consideration for approval by the FDA, get launched, then BioMarin thinks it can achieve non-GAAP profitability in 2017, with profit growth thereafter. If so, then valuation could become less worrisome; however, since we don't know whether the FDA will approve BioMarin's DMD drug, there's a good amount of risk associated with this one that suggests patience is warranted.

No. 2: Incyte Corporation (INCY 0.17%) -- up 46.7%
Enthusiasm over Incyte's drug pipeline has helped propel shares higher this year, but for all the potential tucked away in its clinical-stage drugs, it remains a bit of a one-tricky pony.

The company's only approved medicine on the market is Jakafi, a Janus Kinase Inhibiting drug, or JAK-inhibitor, that is used to treat myelofibrosis and polycythemia vera.

Granted, Jakafi's success so far is nothing to sneeze at. In the second quarter, sales of Jakafi jumped 69% year over year to $142 million leading the company to forecast that its full-year sales will total between $560 million and $575 million. However, despite Jakafi's growth, costly pipeline trials are keeping a lid on profit, so like BioMarin, Incyte isn't a cheap stock. Currently, investors are paying 30 times trailing-12-month sales and 156 times next year earnings to own it.

Justifying Incyte's sky-high valuation may get easier if phase 3 results expected later this year continue to show that its promising rheumatoid arthritis drug baricitinib is safe and effective. Trials to date have been very encouraging, and positive news could help clear the way to an FDA filing and approval. If so, then baricitinib would compete in a market worth billions of dollars annually.

Although baricitinib news could cause shares to move one way or another, its valuation and uncertainty surrounding baricitinib makes this one company that I'll be watching from the sidelines -- at least for now.

No. 3: Regeneron Pharmaceuticals (REGN -0.09%) -- up 41.2%
Since winning approval for its vision restoring drug Eylea, Regeneron Pharmaceuticals shares have been on a tear, returning more than 936% since 2011.

Eylea's market share-winning sales growth shows that investor enthusiasm hasn't been misplaced. Eylea, which is used to treat common causes of vision loss including wet age-related macular degeneration and diabetic macular edema, posted global net sales of $993 million in the second quarter, up 50% from a year ago.

Eylea's ongoing opportunity for growth -- its main competitor Lucentis is still racking up annualized sales of roughly $4 billion -- makes Regeneron compelling, but an even more attractive opportunity may exist for the company's recently approved cholesterol-busting drug Praluent.

Praluent, which was co-developed with Sanofi, won FDA approval in July and the two companies wasted no time in slapping an annual price tag of $14,600 on it. Because Praluent could someday treat millions of people at risk of developing heart disease, the drug has billion-dollar blockbuster potential.

Although Praluent could face competition from drugs that work similarly, industry watchers think it can still help boost Regeneron's EPS to $14.75 next year, up from $12.19 this year.

Because Regeneron already has one top-shelf blockbuster on the market, potentially added another one in Praluent, and its shares sport the best valuation of these three companies, it's the one I'm most interested in still buying.

Looking forward
Of course, there's no guarantee that any of these stocks will go higher or that the drugs in the works or hitting the market will live up to expectations.

Historically, 90% of drugs entering human trials never make it to market and analysts' track record for correctly estimating the peak sales of those that do is, at best, hit or miss, and for that reason, biotech stocks, including these three companies, are suited for the most risk-tolerant investors. For investors willing to accept the risk of buying biotech companies, it's Regeneron that I think is the most compelling.