The best investments aren't always the most obvious ones. The artificial intelligence (AI) boom of 2023 drove share prices through the roof for some of the leading players in this red-hot sector. Meanwhile, a number of less conspicuous AI stocks were left behind for the benefit of astute AI investors.

On that note, these three Motley Fool contributors put their heads together to find some undervalued and nigh-on secret AI stocks for your pleasure and benefit. Read on to see why Atkore (ATKR -0.96%), Sirius XM Holdings (SIRI), and Mastercard (MA 0.72%) are the best AI investments you didn't know you were missing.

The second-largest payments "railway" is predicting a great 2024

Nicholas Rossolillo (Mastercard): Mastercard, much like its larger peer Visa (V 0.62%), just kicked off 2024 in grand fashion. Its digital payments network acts as a global "railway" for money movement. Its data centers, private telecom networks, and cloud-based capabilities help connect banks and financial institutions, merchants, and people in one of the most basic of human communications: transacting business.

The volume of currency Mastercard moved on its network in the 2023 fourth quarter was up 10% year over year to $2.4 trillion, which led to a 9% increase in net revenue from its payments network. Add in another 19% increase from the company's ancillary services (like its cybersecurity and analytics solutions), and total revenue to close out 2023 rose 13% year over year to $6.5 billion.

And Mastercard developed its own generative AI chatbot for its security and fraud detection services, too. So if you're looking for an AI stock for 2024, it checks that box as well.

For the full year 2024, management thinks the global economy will remain in a slower but still healthy growth mode. As a result, the early prediction for full-year revenue is for a low- to mid-teens percentage growth rate over 2023.

The real beauty of buying and holding Mastercard stock for the long term, though, is its established railway for money movement, which is incredibly efficient. As with Visa, the addition of new payment volumes does little to increase Mastercard's operating expenses -- which has led to incredible profit-margin increases over time, and steadily increasing free cash flow.

MA Free Cash Flow Chart

Data by YCharts; TTM = trailing 12 months.

Mastercard stock doesn't come cheap, trading for 26 times one-year-forward expected earnings per share (EPS). But it's been a market-obliterating investment over the long term, and one I'm happy to keep holding indefinitely as its tollbooth-style business keeps raking in the cash from facilitating digital payments around the globe.

You didn't see this coming: Sirius is an AI stock now

Anders Bylund (Sirius XM): I know what you're thinking: Satellite radio veteran Sirius XM isn't an AI stock. That has been true since the beginning of the high-tech entertainment sector, but it's never too late to change. In last week's fourth-quarter earnings call, Sirius unveiled its first direct bet on AI technologies.

When discussing the value that Sirius offers to ad buyers with its large user base and platform-agnostic service, CEO Jennifer Witz dropped this unexpected nugget: "This year, we will focus on expanding advertising services and investing in capabilities, with many powered by generative AI to enhance monetization and efficiency for marketers. In short, we are confident that our strategy in the advertising business is the right one."

Ad sales have supplied a steady 21% to 22% of Sirius' total sales over the last four years. The company chiefly collects ad revenue through its Pandora online music service, but also on satellite radio channels featuring talk shows and other non-music content.

Sirius also offers ad-supported podcasts and an ad-sales platform serving 120 million monthly listeners on third-party audio services.

While it's quite all right to deliver an unmovable revenue trend across the economic turbulence of recent years, Sirius would certainly be better off with a growing revenue stream from ad sales.

I can't wait to hear more about the company's plans to support its advertising with generative AI. Computer-powered narration could deliver audio versions of text ads, for starters. AI can also assist in scheduling and publishing the right spots in the right channels at the perfect time.

Sirius hasn't offered any further details on its AI-based advertising plans, and the company walks a minefield of ethical and technical issues here. But if done right, this effort could elevate advertising from a second-class revenue stream to a leading profit center in a few years.

Meanwhile, Sirius investments will soon be easier to manage after management simplifies its complex stock structure to a single class of shares and revamps its board of directors. And it's hard to bet against investing genius Warren Buffett, whose company holds a grand total of $2.2 billion in Sirius' three stock classes today.

Lastly, the stock is priced to sell. It moved sideways for five years before dropping as much as 41% in 2023. Investors were spooked by slow car sales, limiting the company's most popular listening experience.

But, as fellow Motley Fool Rick Munarriz said when the auto industry started to slow down, Sirius might actually benefit from purported headwinds like rising gas prices. Trading in an old gas-guzzler against a newer and more economical model could put a Sirius-compatible radio in your reach for the first time, after all.

So, Sirius XM Holdings is dipping a digital toe into the AI-driven advertising market, and its audio-service prospects look surprisingly strong. This year could be the right time to finally take a bite of this controversial innovator.

This undervalued construction-materials supplier sees strong growth from the U.S. manufacturing boom

Billy Duberstein (Atkore): With the hypergrowth of artificial intelligence and the passage of the CHIPS Act, it looks like there will be a lot of semiconductor manufacturing capacity built in the U.S. over the next several years.

That's great news for Atkore, a provider of electrical and infrastructure materials, since it supplies the building of a wide swath of nonresidential construction. And what's even better news for investors is that unlike a lot of technology stocks that have run up a lot already, the company trades at a lowly 8.5 times earnings.

Why the discount? Revenue and earnings have been declining, which on the surface doesn't look great. But this is because Atkore benefited from the inflation spike in 2021, when it was able to raise prices on its electrical conduit products and mint record revenue and profits. But now that the inflationary picture is coming back down to normal, the company's pricing is coming down as well.

That's why revenue fell 4.2% last quarter, with adjusted EPS down 8.9% to $4.12. However, that was with a 16% decline in pricing. Atkore actually increased volume by 13% in the quarter on the back of strong U.S. manufacturing growth.

Moreover, Atkore's results beat analyst expectations, and the company actually increased full-year EPS guidance to $17 at the midpoint, up $0.50 from the outlook last quarter.

Even better, Atkore continues to repurchase stock at bargain-basement valuations, with another $96 million in share repurchases last quarter. And the company will pay its first-ever dividend of $0.32 on March 15. That annualizes to $1.28, or about a 0.9% yield at these price levels. While not exactly a high dividend yield, that payout seems set to grow handsomely in the future.

Look for Atkore's stock to potentially turn higher later this year. That's because the pricing headwind the company has seen over the past two years has almost (though not entirely) run its course. And by the second half of this year, just when the summer construction season heats up, pricing declines should be mostly behind the company.

Then, investors will be left with a company growing volumes in the high single digits or low double digits, but with a P/E ratio of 10 or lower.

That certainly seems cheap, and it makes Atkore the rare bargain in the AI ecosystem today.