When the curtain finally closes on 2020, it'll almost certainly go down as one of the most volatile years on record for Wall Street and investors. The uncertainty created by the coronavirus disease 2019 (COVID-19) pandemic initially sent the benchmark S&P 500 lower by 34% during the first quarter. This was followed by the strongest snap-back rally of all-time, with the broad-based S&P 500 ascending to new heights in less than five months.

But just because the stock market has come full circle in six months, it doesn't mean volatility has gone away.

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Last Thursday's market meltdown, which saw the S&P 500 lose roughly 126 points, ranked as the eighth-largest single-day point decline in history. Of course, keep in mind that the 3.5% percentage drop doesn't come close to registering as one of the worst days for the stock market. This move does, however, send a clear message that plenty of economic uncertainty remains, and a stock market crash could occur without warning.

Although stock market crashes can be unnerving in the very short-term, they're actually fantastic news for long-term investors with dry powder at the ready. That's because every single stock market correction in history has eventually been erased by a bull market rally. If investors to choose to buy high-quality companies when the stock market trips up, they usually come out ahead in the long run.

While it's a bit too early to tell whether last week's turbulence will turn into anything resembling a full-fledged correction, here are three great stocks to considering buying when the stock market does crash.

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Teladoc Health

Get the sticks ready, because it's time to beat the drum on telemedicine giant Teladoc Health (TDOC -3.06%), once again.

The beauty of healthcare stocks is that, even if they do get caught up in the emotional whirlwind that is a stock market crash, demand for their products and services remains largely undisrupted. Since we don't get to choose when we get sick or what ailment(s) we develop, cash flow for healthcare companies is pretty steady no matter what's happening with the stock market.

More specific to Teladoc, it's been seeing an incredible build in demand for virtual health visits. Yes, COVID-19 has played a key role in Teladoc's 2020 growth, with visits during the June-ended quarter up an astounding 203% to 2.8 million. But it's not like Teladoc wasn't growing like a weed well before the coronavirus pandemic altered our way of life. The push toward convenience and precision medicine vaulted Teladoc's revenue from $20 million a year in 2013 to $553 million in 2019, long before COVID-19 was declared a pandemic. 

Telemedicine is absolutely a pivotal component of the future of precision medicine. It frees up more time for patient-physician consults, provides consultation flexibility for both parties, and is actually cheaper for insurers to cover than an in-office visit. While we're not going to see in-person trips to the doctor go away, the runway for virtual visits is massive, and Teladoc is still just scratching the tip of the iceberg regarding its potential.

As one final note, Teladoc is in the process of merging with applied health signals provider Livongo Health (LVGO) in a cash-and-stock deal. Livongo utilizes mountains of patient data and artificial intelligence to send tips and nudges to patients with chronic illnesses in order to help them make lasting behavioral changes. It's been working wonders for the company's Diabetes members, and Livongo has already turned the corner to profitability, despite having only a 1.2% share of the U.S. diabetes market. When fully merged, this company is going to be a precision medicine powerhouse.

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AT&T

For more conservative investors who aren't too keen on the short-term volatility that Teladoc would bring to the table, allow me to suggest buying into telecom behemoth AT&T (T 0.71%).

When you think of a basic-need good or service, the idea of buying food, water, or paying for electricity or natural gas probably comes to mind. But how about our dependence on mobile phones? As technology has improved, access to smartphones and wireless technology has brought costs down, making mobile phones something of a basic-need service for many adults in this country. With AT&T's business model predominantly based on subscriptions, a stock market crash is unlikely to have much, if any, impact on its wireless network subscriber count.

Also of note, AT&T is rolling out its first wireless infrastructure upgrades in about a decade. This move to 5G networks isn't going to happen overnight, nor will consumers upgrade their wireless devices all at once. However, this investment in faster download speeds is bound to create a multiyear tech upgrade cycle that'll fuel the high-margin data component of AT&T's wireless segment.

Investors shouldn't overlook the company's streaming opportunity, either. With subsidiary DirecTV continuing to hemorrhage subscribers due to cord-cutting, AT&T is counting on its streaming offerings, HBO Max, and proprietary networks (TNT, TBS, and CNN), to help draw in paying customers. Management's goal is to essentially double worldwide streaming subscribers to HBO Max and HBO (on a combined basis) to approximately 80 million by 2025.

Best of all, patient investors are going to get a 7% dividend yield with AT&T, which is one of the highest, safest yields you'll find. If volatility makes your stomach churn, AT&T is a great stock to park your cash.

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Visa

Another wise idea when the stock market crashes is to buy into payment facilitator Visa (V 0.13%).

As you can imagine, the COVID-19 pandemic has hurt consumer spending and thrust the U.S. economy into its first recession in 11 years. That's bound to reduce the amount of money that consumers are spending, ultimately hurting the merchant fees that payment facilitators like Visa collect.

But here's another way to think about this data. During the Great Recession, Visa saw only one year-on-year decline (2009) in terms of gross dollar value traversing its payment network. Between 2009 and 2018, Visa's share of the U.S. credit card market by purchase volume soared by more than 9 percentage points to 53%, and the amount of purchase volume on its network more than doubled from $764 billion to $1.96 trillion. Visa is the preferred payment processor in the No. 1 economy in the world, which just so happens to be reliant on consumption. 

Visa also solely acts as a cashless payment facilitator. While some of its processing peers also act as lenders, Visa does not lend money. This might seem like a poor choice given the ability to double dip on revenue streams during periods of economic expansion. However, it means that Visa isn't directly exposed to credit delinquencies during inevitable periods of economic contraction or recession. With no need to set aside loan-loss provisions, Visa's profit margin is pretty consistently at or above 50%.

Furthermore, Visa has a ridiculously long runway with which to grow. A majority of the world's transactions are still being conducted in cash, which is providing Visa with an excellent opportunity to court new merchants and wage war on cash in underbanked regions like the Middle East and Africa.

If the stock market crashes, Visa is a great stock to consider buying.