Source: Flickr user Quinn Dombrowski.

Investors approaching retirement shouldn't take many risks with their portfolio, but investors who are younger and who have plenty of time to endure inevitable market drops might want to consider stashing away these top high-risk stock picks from our Motley Fool contributors.

Tim Green: People define risk in different ways, but when it comes to investing in stocks, the biggest risk is that the future doesn't unfold in the way you've assumed. Paying for growth that doesn't ultimately materialize is a surefire way to achieve lackluster returns. Shares of Under Armour (UAA 2.00%), a fast-growing athletic apparel and footwear company, are priced for incredible growth, trading at nearly 90 times last year's earnings, and while this makes the stock extremely risky if this growth doesn't pan out, the potential reward if it does may be worth that risk for some investors.

Under Armour has grown its revenue by more than a factor of 10 over the past decade, recording $3.08 billion of sales in 2014, up 32% compared with the previous year. Nike (NKE -0.01%), the dominant player in the athletic apparel and footwear markets, is still 10 times larger than Under Armour in terms of revenue, but Under Armour is challenging Nike at every turn. Under Armour expects to reach $7.5 billion of annual revenue within three years, driven in part by a surging footwear business. During the latest quarter, Under Armour increased its footwear sales by 61% year over year.

The best-case scenario is that Under Armour becomes the next Nike, and in that case the company's market capitalization could expand by a factor of 5 or more. If this scenario plays out, investors will be handsomely rewarded despite the high price they're paying for the stock.

Todd Campbell: People willing to take on risk in their portfolio might want to consider investing in Portola Pharmaceuticals (PTLA), a small-cap biotech company that could have its first product hit the market next year.

The company has already filed for FDA approval of andexanet alfa, a drug that can reverse the effects of increasingly used factor Xa anticoagulants that are bringing in billions of dollars in annual sales for their manufacturers. Currently, there are no FDA-approved antidotes for these drugs, so an approval could make this drug a staple in hospitals and urgent care centers.

Additionally, Portola Pharmaceuticals is expected to report results from its 7,500 person phase 3 trial of betrixaban, it's own factor Xa drug, in Q1. Betrixaban is being studied against Lovenox, a widely used anti-thrombotic drug that at its peak was hauling in more than $4 billion globally before it lost patent protection.

Because a decision on andexanet alfa and results from the betrixaban study are anticipated soon, 2016 could be a make-or-break year for Portola Pharmaceuticals, and that means it's one high-risk, high-reward stock that ought to be on the radar of risk-tolerant investors.

Matt DiLallo: Given the worsening downturn in the energy sector over the past few weeks, there's a case to be made that energy stocks in general are best suited for high-risk investors. Some are of course riskier than others, including smaller MLPs such as Phillips 66 Partners (PSXP), which fits that category because it lacks the size and scale of its larger peers.

However, what Phillips 66 Partners lacks in size it makes up in growth potential and family ties. That's because the company is an offspring of refining giant Phillips 66 (PSX 0.33%), which created the MLP to drive its midstream growth, the bulk of which is expected to be fueled by dropdown transactions whereby Phillips 66 Partners will acquire fee-based energy midstream infrastructure assets that are already under long-term contracts from its parent in bite-sized pieces. This deal flow is expected to drive robust 30% compound annual distribution growth at Phillips 66 Partners through 2018, without wrecking its balance sheet. 

The risk here is twofold. First, Phillips 66 Partners was created with Phillips 66's best interests in mind. The concern is that it could sell assets to its MLP at values that benefit its interest and not those of MLP investors. Second, given the continued weakness in the energy sector, there's a potential that Phillips 66 and Phillips 66 Partners might pull back on the current plans to grow the Phillips 66 Partners' distribution by 30% per year. Not only is the market not valuing MLP distributions quite as highly as before, but given the 23% slide in Phillips 66 Partners unit price over the past month, it might also not be in investors' best interests to use equity to fund acquisitions. Instead, using internally generated cash flow might be the better option until MLP equity values improve, and that would fuel much slower growth.

Still, for investors with a high risk tolerance, Phillips 66 Partners is an MLP that's could fuel strong total returns, especially if it's able to follow through on its current growth plan.