Figures in this article were udpated on Dec. 22, 2015.

When it comes to long-haul investing, it makes sense to invest in a variety of companies that are operating in different industries. Sometimes, however, figuring out what names are worth including in such portfolios is tricky. In a bid to help, we put our Motley Fool experts to the test, tasking them to offer up their favorite picks for a one-stock portfolio. Read on to learn which stocks they think are the best names to own.

Selena Maranjian
It's hard to think of any company that you could trust enough to be your only stock. Several contenders do come to my mind, but then I remember how, years ago, no one would have believed that Pan Am, General Foods, and Woolworth would fall, or that Sears would be struggling. Thus, I'm going to diversify much of that risk away, choosing the SPDR S&P 500 ETF (NYSEMKT:SPY), an exchange-traded fund, or ETF. ETFs are a bit of a cross between stocks and mutual funds, containing multiple securities, like funds, but trading throughout the day on the market, like stocks, with no minimum required purchase.

This one is basically a low-cost index fund tracking the S&P 500 index, which is made up of 500 of America's biggest and best companies. While there are indeed thousands of publicly traded stocks in the U.S., these 500 actually make up about 80% of the market's overall value, due to their large-cap size. (The weighted average market cap in the ETF is $139 billion.) 

It's also market-cap-weighted, meaning that the component companies are owned in proportion to their market value. Thus, Apple (NASDAQ:AAPL) is the top holding, as its market cap was recently the biggest, near $600 million. 

Want more reasons to like this investment? Well, it's cheap, with a net expense ratio, or annual fee, of just 0.09% -- compared to 1.0% or more for many managed mutual funds. It delivers dividend income, too, recently yielding about 1.9%.

And get this... S&P 500 index funds have outperformed the vast majority of managed mutual funds over many long periods. Simply put, this is just a long-term winner. It will crash on occasion, when the overall market crashes, but the market has always recovered and then gone on to reach new heights. Over many decades, the market has grown by an annual average rate of close to 10%. I'll take that.

Dan Caplinger
Riding Warren Buffett's coattails is an obvious choice, but I'd nevertheless pick Berkshire Hathaway(NYSE:BRK.A) (NYSE:BRK.B) as the one stock I would buy if I could only choose a single company. Even with Buffett having said that investors shouldn't expect the future to bring the same spectacular results that Berkshire has produced throughout its history, Berkshire still has an attractive mix of holdings that puts the Oracle of Omaha's expertise in your corner.

Specifically, wholly owned businesses, like the GEICO insurance unit and the Burlington Northern Santa Fe railroad, have taken advantage of huge opportunities in their respective industries over the years, and they have reliable business models that consistently produce predictable and substantial profits. Meanwhile, Berkshire also owns a blue-chip portfolio of publicly traded securities, with an emphasis on long-term holdings on which the company routinely defers capital gains as long as possible and finds innovative ways to save on taxes.

Finally, Buffett's deal-making prowess is legendary, giving Berkshire investors the benefit of deals that you can't get anywhere else, such as the company's bailout-financing of many struggling companies during and after the financial crisis. Even once Buffett's mortality asserts itself, I think Berkshire will remain a solid holding for decades to come.

Jason Hall
I'd go with Markel Corporation (NYSE:MKL). Much like Berkshire Hathaway, the core of Markel is a very strong and routinely profitable insurance business. Much of Markel's insurance business is largely specialty insurance, which, in many cases, means less competition, and solid profits.

Also like Berkshire, Markel is able to use the float and profits from its insurance operations to invest. Over the past decade, the equity portfolio that Chief Investment Officer Tom Gayner manages has averaged 12.4% annual returns, outperforming the market during that time.

One of the most exciting long-term parts of the story is Markel Ventures, which uses permanent capital, i.e. earnings and not float, to wholly purchase businesses. In much the same way as Warren Buffett uses Berkshire's massive profits to load his "elephant gun," Markel management is building a "baby Berkshire" of operating businesses to provide further cash flows that can be reinvested. Rinse, repeat.

You may be asking, "Why not just buy Berkshire, instead of the copycat?" It really boils down to opportunity. Buffett himself has stated that Berkshire won't be able to grow at the same rate it has in the past. Its $326 billion market cap means that it is simply less nimble. Markel, on the other hand, is still relatively small, with a market cap around $12 billion.

While it may not be as diverse as Berkshire, Markel has a strong balance sheet, fantastic leaders, and is set up for decades of growth at rates Berkshire probably won't be able to match.

Todd Campbell
Owning one stock in a portfolio is a surefire way to guarantee that risk outweighs reward, but if push came to shove, and I had to settle on one company to own, it would be Anthem (NYSE:ANTM).

Every day, roughly 10,000 Americans turn 65, and the aging of America means there is tremendous demand for health-insurance products that can protect retirement nest eggs from the threat of medical bills. As a result, Americans' appetite for products that enhance Medicare is one reason to like Anthem -- but it isn't the only reason that I'm a fan.

The Affordable Care Act's auspicious goal to insure everyone creates even more long-term growth opportunities for the company. Because Anthem offers exchange plans in 14 states, and is one of America's biggest private Medicaid insurers, the company reported adjusted net income of $8.85 per share last year that smoked its January 2013 guidance for $8.

This year, Anthem expects to earn roughly an adjusted $10.15 per share. If it does, then Anthem should have plenty of financial firepower to reward investors. In January, Anthem bumped up its quarterly dividend payout by 43%, to $0.625 per share for 2015. Because aging America and reform could allow Anthem to profit for years to come, it could be the one stock I'd risk my money on.