One of the toughest decisions an investor has to make is what to do with a winning investment. On one hand there's a natural desire to "lock in" gains after a stock has gone on a nice run and redistribute those profits into companies with more potential upside.

On the other hand, many investors have missed out on epic returns because they were content to cash out after a nice 50% gain. As logic would have it, you'll never earn a 1,000% return on a stock if you part ways with it after a 50% bump.

Below, I've highlighted five companies that are all up over 50% in just the past four weeks. I'll let you know what investors should keep an eye on and call one of them out as my pick for being a long-term success.

Big plays in energy
Fuel cells have the ability to take gas, be it petroleum-based or natural, and turn it into an electrical current. Such cells are especially valuable as a supplemental source of energy on an uncertain power grid, or in areas where the grid does not reach.

Aptly named FuelCell Energy (Nasdaq: FCEL) has been on a tear selling such products lately. Thanks in large part to a huge deal with South Korean utility POSCO, FuelCell shares are up 23% over the past month. POSCO's order of 70 MW and 120 MW cells will stretch all the way through 2016. But I won't be taking a bite here, as the company will be forced to compete on a global scale with much larger competitors like General Electric and United Technologies.

Another energy-related play that has seen shares rise precipitously is Frontline (Nasdaq: FRO). The company, which runs a fleet of oil tankers, is certainly bullish on its future. Though some might be concerned about a slow-down in demand from China, a global recovery seems to be taking hold. And with oil over $100 per barrel, demand is certainly evident.

I won't be making a play on Frontline for my All-Star CAPS profile -- but not because it's necessarily a bad investment. Rather, my top pick -- which I'll reveal below -- just has more going for it, even after the large surges all of these companies have enjoyed.

All you need is...drugs?
One of the quickest ways to double an investment -- or see it halved in a short time -- is by investing in pharmaceutical and biopharmaceutical companies. In order for the companies that manufacture these medicines to earn money, they need to have their products approved by the FDA.

That process is no cakewalk, as taking a medicine from idea to research, formulation, clinical trials, and approval can take over a decade. Slip up at any one of these stages and all you've got is a lot of money spent for nothing. That's why word from the FDA often creates a binary event for shares of such companies: The federal "go-ahead" often creates a dramatic increase in share price, while a denial can cause a stock to crater.

Two of our five candidates today were beneficiaries of such events. Shareholders in Corcept Therapeutics (Nasdaq: CORT) were dancing in the streets after the FDA approved Korlym, a drug for the treatment of Cushing's syndrome. Meanwhile, positive news coming from Keryx Biopharmaceuticals (Nasdaq: KERX) and its cancer drug perifosine sent shares up an astounding 40% over the past month.

In both of these cases, investors need to be careful. A pharma company's stock can come down just as swiftly as it went up. And approval itself is not a sure-fire sign of the product's market success.

Familiarity with the industry you're investing in is always necessary, but it's even more paramount with drugmakers. I'm no doctor, nor am I intimately familiar with either product. I would take my money and run, given the risk involved and my lack of competence, so I won't be making a CAPScall on either company.

Let's get down and dirty
Though all five of these companies I'm covering here may continue their solid run, if I had to pick one to invest in over the next three to five years, it would be Veolia Environnement (NYSE: VE). Back in December, I dubbed it my top dividend pick for 2012, and it certainly hasn't disappointed, as shares are up over 50% on the year.

Does that mean that Veolia -- which focuses on waste and water treatment -- is a slam-dunk investment? Hardly. Under the previous CEO, the company expanded way beyond its capacity and was left with a crushing debt load.

But current CEO Antoine Frerot has dedicated himself to scaling back Veolia's presence and focusing it far more keenly on its core competencies. Given the certainty for demand in waste and water treatment for the foreseeable future, I consider Veolia to be the best bet of the bunch -- as long as Frerot and his plan remain intact.

There's another skyrocketing stock that I would suggest investing in. We've created a special free report to reveal what company it is: "Discover the Next Rule-Breaking Multibagger." Inside the report, you'll find out about a company that, though it's up 225% over the past two years, still has tons of room for growth. To find out what company this is, get your copy of the report today, absolutely free!