Finding stocks that have what it takes to be mainstays in your portfolio for the long haul may seem like a big ask. And in the current market environment, with many top stocks trading at incredibly high valuations and fears about another imminent market crash running wild, investors are getting increasingly pickier about where they put their hard-earned cash.

If you're looking to add more high-quality stocks to your basket this month, keep on reading. Here are two smart buys with plenty of upside potential to spare that can generate substantial long-term portfolio growth for investors.

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1. Teladoc

The global telemedicine field already represents a multibillion-dollar market, and analysts think the industry is just beginning to realize its long-term growth potential. According to a market analysis report by Grand View Research, the telehealth market is on track to hit a compound annual growth rate of more than 22% between this year and 2028.

Another study by Facts & Factors attributes this rapid rate of growth to a range of stimuli, including "an increased adoption rate of telehealth technology in the field of radiology, cardiology, and behavioral health" and "the increasing rate of elective surgeries and reduced hospital visits for regular checkups." The study also cited population growth and an increased rate of patients suffering from chronic illnesses as other driving factors for the overall expansion of the global telehealth industry.   

And at the forefront of this lucrative industry, which was valued at just shy of $56 billion in 2020, is Teladoc (NYSE:TDOC). Although shares of Teladoc have dropped since its hot streak in the early days of the pandemic -- the stock is down 25% year to date -- the company continues to deliver remarkable revenue growth quarter after quarter. In its most recent quarterly report, management said that the company's revenue increased 151% compared to the first quarter of 2020. Meanwhile, visits on the platform grew by a rate of 56% from the year-ago quarter.

Teladoc has an exceptional track record of revenue growth. For instance, in 2016, 2017, 2018, and 2019, the company reported respective revenue increases of 59%, 89%, 79%, and 32%. The pandemic fueled blistering demand for Teladoc's platform and services, causing its revenue to surge 98% in 2020 over the previous year.

Teladoc's share price decline over the past several months shouldn't discourage investors from jumping on board -- in fact, its long-term growth potential in the explosive industry it dominates coupled with its robust financials are two great reasons to hit the buy button on this growth stock while it's still trading at a discount. As Warren Buffett says, "Whether we're talking about socks or stocks, I like buying quality merchandise when it is marked down."

2. Airbnb

The vacation rental market is another multibillion-dollar industry poised for massive growth in the coming years. Grand View Research reports that from a global valuation of more than $87 billion in 2019, the space is poised to realize a valuation of nearly $114 billion as of 2027 -- and that's despite the impact of the coronavirus pandemic.

As of 2019, Airbnb (NASDAQ:ABNB) controlled a roughly 20% share of the vacation rental market in the U.S. Airbnb's initial public offering last December marked one of the biggest of the year, with the company's valuation surpassing $100 billion on its first day as a publicly traded entity. Since that time, Airbnb's market capitalization has retracted to just over $80 billion, with shares trading down about 5% from the beginning of the year.

Does this mean long-term investors should shy away from buying Airbnb stock? Absolutely not. It's no secret that the travel industry has been hit incredibly hard by the pandemic. Last year, Airbnb's revenue and earnings were noticeably affected by these broader trends. While its first-quarter report made it apparent that the company's recovery from pandemic headwinds was well under way, modest year-over-year revenue growth caused some investors to react negatively and sell off shares, driving the stock's price down nearly 19% in the month of May.

That first-quarter report offered plenty of encouraging signs. For instance, while at first glance the company's year-over-year revenue growth of 5% wasn't much to write home about, management said that the company generated more revenue during the three-month period than it had in either the first quarter of 2019 or 2020. And where Airbnb had reported negative adjusted EBITDA of $334 million in the first quarter of 2020, this metric improved to a negative $59 million in the first quarter of 2021.

While shares of Airbnb haven't made any impressive headway lately, one could argue that the stock's recent retraction was justified, as it still trades at a relatively steep valuation of about 24 times sales. That said, analysts think the stock has a high upside potential -- nearly 70% at the time of this writing -- so Airbnb has plenty of runway left. If you're looking to invest in a stock with durable long-term growth potential and a strong competitive advantage in a thriving industry, Airbnb is a no-brainer pick for the shrewd investor.

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.