Investing in 2020 should require a seatbelt and helmet, because it's been an exceptionally bumpy ride.

The uncertainty and panic created by the coronavirus disease 2019 (COVID-19) pandemic initially walloped equities and sent the benchmark S&P 500 lower by a whopping 34% in 33 calendar days. Amazingly, it took less than five months for the broad-based index to regain everything back that was lost. Both the steepness of the more than 30% decline and the swiftness of the rebound represent all-time records.

But as we've learned throughout September, volatility hasn't gone away. There are plenty of catalysts fanning the flames for a potential stock market crash or correction at some point in the future.

Of course, a crash or correction doesn't have to be a bad thing. If you're a long-term investor with cash on hand, a falling stock market represents an opportunity to buy into high-quality and innovative businesses at a perceived discount. If your investment horizon is long enough, corrections should be welcomed with open arms.

Most important, you don't have to be wealthy to become rich investing in the stock market. If you have $1,000 that won't be needed for bills or to cover emergencies, that's more than enough capital to buy the following top stocks during a market decline.

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Exelixis

There are literally hundreds of biotech companies that investors can choose to buy. The vast majority of them aren't making money, or if they are, they're priced at an aggressive premium. Then there's cancer-drug developer Exelixis (NASDAQ:EXEL), which has one of the lowest PEG ratios in the entire industry, which is reflective of its relatively low price-to-earnings ratio and double-digit growth rate.

The way I view it, there are three big selling points for Exelixis. First, you have Cabometyx, the company's lead drug that looks to be well on its way to topping $1 billion in annual sales. Cabometyx is approved in first-and-second-line renal cell carcinoma (RCC), as well as advanced hepatocellular carcinoma. Cabometyx ran circles around first-line RCC treatment Sutent in clinical studies, and it had it little trouble securing second-line RCC share.

Secondly, Exelixis has an expansive pipeline. Flush with cash, the company has reigniting its internal research engine, and it's been partnering its in-house developed drugs, along with Cabometyx, in a host of cancer-focused preclinical and clinical studies. According to Exelixis' pipeline page for Cabometyx, there are more than six dozen ongoing clinical trials involving its lead drug as a monotherapy or combination treatment

Finally, there's a real chance of a buyout at some point. We just watched Gilead Sciences gobble up Immunomedics for an absurd $21 billion. If the very profitable Exelixis, which has almost $1.2 billion in net cash and is generating around $400 million in operating cash flow a year, is only worth $8.3 billion, something is amiss. If the stock market drops, Exelixis is a go-to healthcare stock to buy. 

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NextEra Energy

I know -- the idea of investing in utility stocks, unless you're a highly conservative, income-seeking investor, probably sounds as boring as watching paint dry. But I promise you, electric utility NextEra Energy (NYSE:NEE) is making it worth investors' while.

The NextEra secret to success has been the company's aggressive shift away from fossil fuels and toward renewable sources of energy. Make no mistake about it, the upfront cost of renewable energy projects can be costly. But NextEra is benefiting from historically low lending rates, as well as the fact that electricity generation costs from renewable energy sources are considerably lower than that from fossil fuels. These low generation costs are what have allowed for a high single-digit compound annual earnings growth rate. As of today, no U.S. utility generates more capacity from solar and wind power than NextEra Energy.

For NextEra's more traditional utility operations, investors enjoy plenty of transparency. That's because the company's traditional electric generation is overseen by state-run energy commissions. Though NextEra can't simply pass along price hikes any time it pleases, being regulated means the company also avoids potentially volatile and unpredictable wholesale electricity pricing.

Currently working on an 11-year winning streak (as measured by total return, inclusive of dividends paid), NextEra is a fantastic stock to add on any market weakness.

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Shopify

If growth is more your thing, then I'd strongly suggest taking your $1,000 and investing it into cloud-based e-commerce solutions provider Shopify (NYSE:SHOP).

Prior to COVID-19, we were witnessing a steady transition of businesses moving online. Since the pandemic, we've observed an incredible uptick in online consumerism as the traditional office environment has been disrupted. This has taken Shopify's e-commerce solutions, which are designed to improve supply chain management and streamline sales, and completely supercharged demand. In the COVID-19-challenged second quarter, Shopify saw a 71% increase in new stores created from the sequential quarter, with gross merchandise volume on its platforms catapulting 119% from the prior-year quarter. 

With a market cap of $104.5 billion, Shopify isn't going to come across as the same type of fundamental value as, say, NextEra or Exelixis. But it more than makes up for its premium with superior growth potential. Shopify's total addressable market just among smaller businesses (i.e., those with less than 500 employees) is $78 billion. Plus, the company has begun to see a big uptick in usage by larger, brand-name businesses.

Best of all, 86% of its revenue in the most recent quarter was derived from its core Shopify or Shopify Plus platforms. These are monthly subscription services that are responsible for highly predictable revenue. 

Assuming Shopify continues to snag new merchants, Wall Street is counting on sales to skyrocket from $1.58 billion 2019 to $17.95 billion by 2024. As I said, it might look pricey now, but Shopify could be one heck of a bargain, and the perfect high-growth stock to buy in a falling market.