You wouldn't know it by looking at the year-to-date performances of the major U.S. indexes, but this has been one of the most volatile years in history. The broad-based S&P 500 set a record for the fastest descent into bear market territory in the first quarter, and it countered this by staging the quickest rally from a bear market bottom to a new high over the past five months.

Despite this volatility unnerving a number of investors, it's actually a good thing. You see, the stock market has a history of heading higher over the long run as operating earnings expand. This means every previous stock market correction or bear market have proved to be an excellent buying opportunity for investors.

The thing is, anytime can be the perfect time to put your money to work if you're investing time horizon is measured in years. If you have, say, $2,000 that won't be needed to pay bills or cover emergencies, you have more than enough cash to invest in these three unstoppable stocks right now.

A vintage stopwatch lying atop a pile of one hundred dollar bills.

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Mastercard

In the financial space, one stock that seemingly can't be held back for any significant length of time is payment facilitator Mastercard (NYSE:MA). Shares of Mastercard are up nearly 1,700% over the trailing 10-year period, but still offer incredible upside to patient investors.

The beauty of the Mastercard model is threefold. First, Mastercard isn't a lender. Instead, it solely acts as payment facilitator for credit and debit transactions. Though this means it's unable to double dip by generating interest income and merchant fees during an expanding economy, it also ensures that the company isn't directly exposed to loan delinquencies during economic contractions and recessions. This lack of bad loan exposure is a key reason Mastercard's profit margin is regularly around 45%.

Secondly, Mastercard is a business model that allows you to bet on the well-being of the U.S. and global economy. Even though recessions are an inevitable part of the economic cycle, the U.S. and global economy spend a far more time expanding than contracting. This allows Mastercard's sales and profits to grow for years at a time. And as an added bonus, Mastercard holds the second-highest share of credit card network purchase volume in the consumption-driven U.S. market.

Third and finally, Mastercard is still just scratching the surface in terms of cashless transactions. With more than 80% of global transactions still being conducted in cash, Mastercard has an exceptionally long runway with which to convert cash buyers into credit or debit-card users. There's ample opportunity in Southeast Asia, the Middle East, and Africa for Mastercard to potentially grow its business by a high single-digit to low double-digit rate for a long time to come. 

A surgeon holding a one dollar bill with surgical forceps.

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Intuitive Surgical

Another absolutely unstoppable stock that you can confidently buy now if you have a long-term investing horizon is surgical systems developer Intuitive Surgical (NASDAQ:ISRG).

One of the more amazing things about Intuitive Surgical is the company's dominant presence in the robotic-assisted surgical setting. As of the June-ended quarter, it had 5,764 of its da Vinci surgical systems installed worldwide. That's far more than any of its competitors on a combined basis. This makes the da Vinci system the de facto choice for hospitals and surgical centers and virtually eliminates any concern about client churn. 

Perhaps the most alluring aspect of Intuitive Surgical is that it's a razor-and-blade-modeled business. The company's da Vinci system, while pricey ($0.5 million to $2.5 million per system), generates only mediocre margins due to it being so intricate to build. Where the company generates the bulk of its margin is from selling instruments and accessories with each procedure, and from regularly servicing its installed systems. The da Vinci system is the razor, while the high-margin instruments and servicing are the blades. As the number of installed systems increases worldwide, the percentage of revenue generated by these higher-margin channels continues to grow.

Also, don't overlook the fact that Intuitive's soft tissue surgical system has plenty of surgical market share it can acquire. Though it's already the dominant system used for urology and gynecology surgeries, the company's management believes the da Vinci system has a solid future as a major player in thoracic, colorectal, and general soft tissue surgical procedures.

In other words, a double digit growth rate is a very realistic expectation throughout the decade, if not longer.

Three wind turbines next to an electrical tower at sunrise.

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NextEra Energy

Finally, when you think unstoppable, you think of... an electric utility stock? I know it might sound odd, but NextEra Energy (NYSE:NEE) offers everything investors could ever want in an unstoppable stock.

The most obvious lure of utility stocks is the high level of demand predictability associated with them. This is to say that if you own a home or rent, you need basic utility services like electricity or natural gas and water. No matter how well or poorly the economy is performing, demand for these basic-need services doesn't change very much. This predictability plays a key role in helping utilities decide how much money they want to outlay for capital projects (i.e., equipment upkeep, upgrades, and expansion).

But what sets NextEra Energy apart is the company's focus on renewable energy. No electric utility generates more capacity from solar or wind than NextEra. On the downside, these have been pricey upfront investments. But the payoff over the long run is plain as day. NextEra's energy generation costs should be well below its peers, resulting in superior profit growth in an otherwise slow-growing sector.

It also doesn't hurt that the Federal Reserve is choosing to keep lending rates at a record-tying low. Should NextEra continue to invest aggressively in solar and wind projects, it'll be able to do so while paying minimal interest.

To add, NextEra's traditional electric operations are regulated. This is a fancy way of saying that it can't raise rates on its customers without the approval of a state utility commission. That might sound like a hindrance, but it's actually good news because it means NextEra isn't exposed to potentially volatile wholesale electricity pricing.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.