There are three types of biotech stocks to buy that generally fall into one of these typically non-overlapping groups:

Value investing isn't usually associated with biotech stocks to buy; if anything, biotech investors are notorious for being overly optimistic. But occasionally, you can find a diamond in the rough that can bounce back.

Left for dead
A few years ago, Facet Biotech, the drug discovery spinoff of PDL Biopharma, was trading at less than the value of its cash on hand. A ways away from getting a drug on the market, the biotech didn't look like a stock to buy since it had to spend the cash to develop the drug.

Except its partner, Biogen Idec (BIIB -0.72%), saw value and decided to make an unsolicited bid to buy the company. Fast forward a few months, and Abbott Labs topped Biogen's offer at triple where the biotech stood before Biogen expressed its love. Investors willing to buy stock in the unloved company made out like bandits.

Beaten down biotech stocks to buy
I did a screen for companies trading at least 50% off their 12-month high, and I set the minimum market cap at $100 million to avoid companies that were not only left for dead, but already rotting.

Of the 29 potential companies, here are three biotech stocks that could be buys if the companies can turn things around.

Amarin (AMRN -1.20%) is a stock to buy if it can find a partner or, better yet, a buyer for its fish oil drug Vascepa. Of course, we've been saying that for nearly a year. AstraZeneca's purchase of Omthera Pharmaceuticals for $323 million plus $120 million in potential milestone payments didn't exactly help the company's valuation. Amarin will be hard-pressed to find a buyer above its current valuation, leaving a partnership -- perhaps with a potential sale down the line -- as the most obvious choice. To be a stock to buy, Amarin needs a big pharma company willing to endorse Vascepa's potential.

Geron (GERN -2.61%), the stem-cell-turned-oncology company, sank last year after both its cancer pipeline drugs failed. The company believes one of them, imetelstat, may still be worthy of further development given that data from another trial was actually positive. The company plans to wait for an investigator-sponsored trial in myelofibrosis to read out before proceeding.

Idenix Pharmaceuticals (NASDAQ: IDIX) has had a rough year, with two of its hepatitis C drugs getting put on a clinical hold, and eventually being put out to pasture. Fortunately they weren't the only drugs in Idenix's arsenal; samatasvir, formerly IDX719, is partnered with Johnson & Johnson (JNJ 0.29%) to help create an all-oral combination. Just yesterday, the companies announced the start of a phase 2 trial testing samatasvir with Johnson & Johnson's simeprevir.

Best biotech stock to buy?
It's a tough call between the three. Amarin has an unknown risk -- whether mangement will find a partner -- which is hard to quantify. Unless it gives away the farm, if Amarin finds a partner, shares are likely to soar. If not, look out below.

The history of Geron and its struggles makes it hard to recommend it as a stock to buy. I think imetelstat might turn out to be active, but Geron has a long road ahead of it. There's plenty of time for investors to buy in.

That leaves Idenix, which is probably the least risky of the three in terms of a path forward. Samatasvir likely works, but Idenix might be so late to the party that samatasvir is essentially irrelevant by the time it hits the market. Johnson & Johnson also has multiple non-exclusive partners, so there's potential to be left at the altar if the pharma giant finds a better mate.