You should seriously consider adding a few biotech stocks to your portfolio if you don't already own some. Why? Because they give you an opportunity to make higher returns.

And the good news is that you don't have to bet on speculative clinical-stage biotechs. Several established biotechs have performed really well over the past few years and should continue to do so in the future. Here's why Celgene (NASDAQ:CELG), Exelixis (NASDAQ:EXEL), and Vertex Pharmaceuticals (NASDAQ:VRTX) are three top biotech stocks you can buy right now.

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Celgene

I viewed Celgene as one of the best biotech stocks on the market before the company announced the failure of GED-0301 in a late-stage study targeting treatment of Crohn's disease. Even after this clinical setback, I still think Celgene is one of the best biotech stocks on the market -- except now it's an even greater bargain than before.

There's a long list of things to like about Celgene. Its top drug, Revlimid, continues to enjoy solid momentum. Sales for another blood cancer drug, Pomalyst, are growing even faster. Psoriasis and psoriatic arthritis drug Otezla is Celgene's fastest-rising product.

What about the pipeline after the GED-0301 disappointment? Celgene still has one of the most promising candidates around in ozanimod. The drug is being evaluated in late-stage studies for treating relapsed multiple sclerosis and ulcerative colitis. Celgene also appears to be in good position to extend its dominance in the blood cancer space with late-stage candidates CC-486 and luspatercept.

Thanks to the sell-off following the GED-0301 results, Celgene stock now trades at less than 14 times expected earnings. I still expect the company to generate annual earnings growth of 20% or more over the next five years. That kind of growth at Celgene's valuation makes buying this biotech stock an easy call, in my opinion. 

Exelixis

It's been pretty much nothing but good news for Exelixis lately. The biotech's Cabometyx (cabozantinib) is rapidly gaining ground as a therapy for previously treated renal cell carcinoma (RCC). Exelixis recently filed for approval of the drug as a first-line treatment for RCC based on outstanding phase 2 results from the Cabosun trial. The FDA even granted priority review status, which means the agency should make a decision on approval for Cabometyx in the additional indication by Feb. 15.

Last week, Exelixis added to the growing list of positive developments with its announcement that Cabometyx met the primary endpoint in the late-stage Celestial study targeting treatment of advanced hepatocellular carcinoma (HCC). The study was stopped after a second interim analysis determined that there wasn't a need to continue after the drug demonstrated clear efficacy. Exelixis plans to submit for approval in the HCC indication in the first quarter of 2018.

Success for Cabometyx helped Exelixis become profitable for the first time ever earlier this year. The biotech also paid off its debt and socked away nearly $350 million in cash, cash equivalents, and short-term investments. That puts Exelixis in great shape to add to its pipeline with licensing deals or acquisitions, something the company's management has said it wants to do.

Unlike Celgene, Exelixis stock doesn't look cheap by any stretch of the imagination. Shares trade at 40 times expected earnings. However, with tremendous prospects for Cabometyx in the RCC and HCC indications -- along with the potential for the drug to be a key component of combinations for treating other types of cancer -- this biotech stock still has plenty of room to move higher.

Vertex Pharmaceuticals

Vertex Pharmaceuticals has also enjoyed a streak of good news. The biotech announced positive results from a couple of late-stage studies of its tezacaftor/ivacaftor combination in treating cystic fibrosis (CF) in March. An FDA decision is expected by Feb. 28. In May and in August, Vertex picked up additional FDA approvals for CF drug Kalydeco. The company also secured reimbursement deals for its other CF drug, Orkambi, in Ireland and Italy over the summer. 

But perhaps the best news of all for Vertex came in July. The biotech announced positive results from phase 1 and phase 2 studies of three different triple-combination CF regimens. Vertex plans to initiate pivotal late-stage studies for two of these combos in the first half of 2018. 

What all of this means is that Vertex is steadily advancing toward its goal of providing therapies that can treat all CF patients. Approval of the tezacaftor/ivacaftor combo would bump the number of potential patients up to around 44,000. The triple-drug combos could increase Vertex's target population up to 68,000. That would leave only 7,000 or so patients without an approved therapy -- and Vertex has pipeline candidates that hold potential for treating those individuals also. 

At first glance, Vertex looks ridiculously expensive, with the stock trading at nearly 49 times expected earnings. But with good chances for its two-drug and three-drug combos to reach the market, Wall Street analysts think the biotech can grow earnings by close to 70% annually over the next few years. These tremendous growth prospects seem attainable, in my view, making Vertex a great stock to buy now.

Keith Speights owns shares of Celgene. The Motley Fool owns shares of and recommends Celgene and Exelixis. The Motley Fool recommends Vertex Pharmaceuticals. The Motley Fool has a disclosure policy.