Long-term investing has been shown to outperform short-term trading, but that doesn't mean that every stock deserves a spot in set-it-and-forget-it portfolios. Many of today's winners won't have what it takes to thrive for decades, so it's critical to due your homework and focus on those companies best positioned for long-haul success. To help you find the best long-term stocks to buy, we asked three Motley Fool contributors to scour their universes. Read on to find out why they think American Tower (NYSE:AMT), Canopy Growth (NYSE:CGC), and Alphabet (NASDAQ:GOOG) (NASDAQ:GOOGL) are top stocks that long-term investors should stash away.

Riding the wave of global internet growth with...real estate?

Tyler Crowe (American Tower): Investing in growth stocks over a multidecade period is incredibly hard because so few businesses are able to maintain their competitive advantages for that long. Two things tend to remain constant over time, though: improving standards of living in emerging markets and real estate.

Hands arranging cut-up pieces of construction paper into an upwardly trending arrow.

IMAGE SOURCE: GETTY IMAGES.

That combination of factors is what makes American Tower a compelling growth stock for the next half century. Don't let the name mislead you; the owner and operator of cellular towers isn't a pure play on the U.S. Rather, it has a diverse portfolio of towers and structures for telecommunications equipment across 19 countries on four continents. In fact, less than a quarter of the company's (technically, it's a real estate investment trust, or REIT) assets are in the U.S.

As mobile data speeds become faster, data transmission requires a more dense network of communication equipment. Therefore, American Tower benefits from more tenants renting space for their equipment on tower sites. This is providing ample room to grow in more mature markets such as the U.S. and Germany as they move from 4G to 5G as well as longer runways in emerging markets such as Nigeria and India as they move from 3G to 4G to 5G. According to management, smartphone data usage will grow 31% annually worldwide through at least 2023.

For American Tower investors, this has translated into stellar growth. Over the past decade, it has grown its free cash flow by more than 15% annually. The company is also making a large push into emerging markets in India, Latin America, and Africa, where it can get in on the ground floor of exponential data usage growth over the next several decades.

Whether cellular towers will remain the go-to method for communication equipment for the next half century remains to be seen, but it seems like a reasonable bet today, and it makes American Tower a very compelling growth stock for such a long time horizon.

A marijuana leaf.

IMAGE SOURCE: GETTY IMAGES.

A top pot stock for the long haul

Todd Campbell (Canopy Growth): It could be a great time to add Canopy Growth, Canada's largest cannabis company, to long-haul portfolios. Why? Because shares in Canopy Growth -- the biggest pot stock by revenue, market cap, and cash on the balance sheet -- have tumbled to levels that could make buying it savvy given global legalization trends and estimates that the global marijuana market could be worth $200 billion in 10 years, according to Stifel, an investment research firm.

Canopy Growth reported its latest quarterly results on Aug. 14, and this update was particularly important because it's the first one since Canopy's board forced marijuana pioneer Bruce Linton out of its C-suite. Fortunately, Canopy Growth's financials show it's still a dominant player. Net revenue was 90.5 million Canadian dollars, up 249% year over year, and its recreational cannabis revenue was CA$50.4 million. That keeps it neck and neck with top competitor Aurora Cannabis (NYSE:ACB), which reported preliminary quarterly net sales of CA$100 million to CA$107 million on Aug. 6.

Under Linton, Canopy Growth announced in January it's investing up to $150 million to create a hemp industrial park in New York targeting the U.S. cannabidiol (CBD) market. He followed that up with an innovative agreement in April to exchange $300 million for an option to acquire Acreage Holdings (OTC:ACRGF) upon legalization of marijuana in the U.S. at the federal level. If Canopy Growth exercises its option, Acreage Holdings investors will collect 0.58 shares in Canopy Growth for each share of Acreage Holdings they own.

Linton’s aggressive investments and a policy of rewarding employees with significant, profit-zapping stock options could be why he lost his job. But he's positioned Canopy nicely to capitalize on this multibillion-dollar market. Canopy's still on track to launch cannabidiol-based products in the U.S. later this year, and a rollout of marijuana consumer goods products such as edibles in Canada is anticipated in December. Given that those represent huge market opportunities, buying Canopy Growth shares for long-term portfolios while they’re on sale could be smart.

The tech giant that can't be stopped

Travis Hoium (Alphabet): There aren't a lot of companies I would want to own for the next 50 years given the disruption taking place in the economy today. Manufacturing is under pressure, energy is changing, and retail is being turned on its head. However, one company that has an upper hand in almost every business it enters is Alphabet, the search giant that now does everything from email to cloud hosting.

You can see the rapid growth of Alphabet's business over the past decade and the $35 billion of net income that it earned over the past year. What that doesn't show is how much power the tech giant has to expand its business and fend off upstarts that might be its competition.

GOOG Revenue (TTM) Chart

GOOG Revenue (TTM) data by YCharts.

Alphabet has used its dominant position in search to enter email, maps, mobile and PC operating systems, voice recognition software, and many other products. The slow creep of products that Google makes that ingrain it more and more in people's lives also comes with data that makes search and other technology better. It's a feedback loop that gets bigger and bigger every year.

I don't think there's any end in sight to technology and software becoming more and more entrenched in our lives. Alphabet will continue to play an outsized role in that transition, and it's proven it has the ability to turn its technology into massive profits. That's the kind of business I want to own for the next 50 years.