Owning just a single stock in your portfolio is a risk that most investors should never run. However, in putting together a diversified portfolio, focusing on the best stocks available in the market right now is a great way to find candidates that will stand the test of time and deliver the long-term results you need. With that in mind, below you'll find three stocks that our Motley Fool contributors have made the case for as the single best stock to own today: Johnson & Johnson (JNJ -0.02%)Costco Wholesale (COST 1.00%), and Wells Fargo (WFC 1.25%).

Sean Williams (Johnson & Johnson): With the stock market hiccuping on a near-regular basis, the best stock to own today, in my opinion, is healthcare conglomerate Johnson & Johnson.

Three things, in particular, make it a great stock to own.

First, it's an extremely diverse company. It has a consumer healthcare segment that doesn't grow very fast, but does maintain very predictable cash flow. Similarly, its medical device franchise is growing slowly now, but provides long-tail accelerated growth opportunities as access to medical care globally improves and life expectancies lengthen. Finally, we have J&J's pharmaceutical segment, which offers the most risk from patent expirations, but also delivers superior margins and the quickest potential for growth.

Second, J&J markets a lot of inelastic goods. If the economy is shrinking, people are still going to buy Band-Aids, toothpaste, Tylenol, and so on. Consumers also can't decide when they're going to get sick and for that matter, what illness they're going to deal with. A recession doesn't mean the latest cancer drug gets a price reduction or that a spinal implant is now 25% off. Because of the innovation that goes into the development of J&J's products, it's generally impervious to pricing pressures, and thus we see its share price only minimally affected by stock market downturns.

Finally, we have J&J's rich history. It's delivered adjusted earnings-per-share growth in 31 consecutive years, and has a 53-year streak of boosting its annual dividend. Among the 7,000-plus publicly traded companies, it's one of only three to still bear a AAA-credit rating according to Standard & Poor's. This last point suggests the confidence S&P has in J&J to pay back its $20 billion in debt.

With J&J, you get an above-market dividend yield of 3.1%, as well as steady growth on the top and bottom lines. I believe you'll struggle to find a more attractive stock worth owning today.
Andres Cardenal (Costco Wholesale): In times of increasing market volatility and economic uncertainty, investors typically want to focus their attention on companies that are solid enough to produce consistent earnings and cash flows through good economic times and bad, and Costco truly excels when it comes to fundamental soundness.

The retail industry is notoriously challenging and competitive. However, Costco is a unique player in the business, thanks to its differentiated business model, which protects the company from its rivals and generates consistently strong financial performance for investors.

Image source: Costco.

The company makes most of its profits from membership fees, not margins on sales prices. This allows Costco to sell its products at aggressively low prices, a major source of competitive advantage in the discount retail industry. Since profits don't depend on sales volume too much, this means that it produces consistent financial performance through the ups and downs of the business cycle.

Customers are remarkably loyal to Costco: Its membership renewal rate was 88% on a worldwide basis last quarter, reaching 91% in big markets such as the U.S. and Canada. New membership signups increased 7% year over year, and the company ended the quarter with a big customer base of 45.4 million members.

It's hard to tell how the stock market or the global economy will evolve this year, but Costco is certainly strong enough to continue delivering healthy performance in 2016 and beyond.

Dan Caplinger (Wells Fargo): Rising interest rates courtesy of the Federal Reserve have contributed to the nervousness in the stock market lately, but the financial industry tends to benefit from a rising-rate environment. That puts Wells Fargo in a prime position to build on its long-term performance with gains in the years to come.

Many big banks have been moving away from gathering assets, instead seeking to downsize in order to avoid greater regulatory scrutiny and to make their balance sheets look more attractive. But Wells Fargo has gone in the opposite direction, becoming the third-largest bank by assets as it continues to follow its business model of making prudent and conservative yet lucrative loans to consumers and commercial customers alike. Its track record of avoiding some of the excesses that its peers experienced during the financial crisis has only added to its reputation.

Now, the prospect of higher interest rates could boost net interest margins and allow Wells Fargo to become even more profitable. Rising mortgage rates could offset some of those gains, but overall, Wells Fargo is in a great position to benefit from the current financial environment and sustain its excellence this year and far into the future.