Stock indexes have slipped into a bear market, and companies are suffering from higher inflation. But the good news is that this is a temporary situation. There are still amazing stocks out there that could boost your portfolio over the long term. In fact, considering recent declines in valuation, many stocks make great buys today.

It's always best to invest in several stocks -- never bet on just one or two players. I've already done this and plan on hanging on to my investments for the long term, but today, if I could add only one stock to my existing portfolio, it would be a company that leads in not just one, but in two big businesses. I'm talking about Amazon (AMZN -0.68%).

A global leader in two businesses

I already own shares of Amazon but would be happy to buy more of this champion. Amazon is a global leader in both e-commerce and cloud computing. These markets are set to grow in the double digits in the coming years.

Amazon's e-commerce business is suffering today, due to the current economic environment. For instance, the company's logistics and transport costs have climbed, due to rising inflation. But as mentioned earlier, this situation won't last forever.

Meanwhile, Amazon has made progress managing these headwinds. The company cut costs that are within its control and improved productivity within its fulfillment network. For the past few quarters, operating cash flow and operating income have decreased. Recovery probably will take some time.

But the long-term picture looks bright. Amazon has grown its Prime membership numbers to more than 200 million customers worldwide. And these members are spending more and relying on Amazon for more and more services. This should drive growth down the road.

Double-digit gains

Now let's turn to cloud computing. Amazon Web Services (AWS) is the global leader in this area. And importantly, the current economic environment hasn't hurt AWS' business. AWS sales and operating income have both continued to rise in the double digits. AWS continues to expand its presence worldwide and win new projects. For example, Delta Air Lines chose AWS to transform its digital platform.

The health of AWS is particularly important for Amazon because it's a key profit driver for the company. Last year, AWS represented more than 70% of Amazon's total operating income. Today, this strength will help Amazon through difficult times. And tomorrow, it should drive growth.

Amazon also has the resources necessary to venture into other businesses that could add to growth in the future. It's building its presence in healthcare, for example. As part of this, Amazon plans on buying One Medical. The primary-care organization combines in-person care and telemedicine in locations throughout the U.S.

This opens the door to another high-growth market for Amazon. Grand View Research predicts a compound annual growth rate of more than 44% for the U.S. telemedicine market through 2028.

Why buy now?

All of this sounds good, but why is Amazon a buy right now? We're at a bit of a lull in Amazon's stock performance, and that's resulted in a lower valuation. Amazon is trading at around its lowest price, in relation to sales, in five years. At the same time, revenue has continued to climb. Considering Amazon's long-term potential, this looks like a great entry point.

AMZN PS Ratio Chart

AMZN PS Ratio data by YCharts.

It may take some time, of course, for Amazon's earnings and share price to bounce back. Today's headwinds may persist for a while, and investors may wait for clear signs of earnings improvement before returning to Amazon. It's impossible to time the market and get in on a stock at the very lowest price.

It's best to seize the opportunity when valuations looks reasonable -- and there's reason to be positive about a company's business over the long term. That's why, right now, if I could buy only one stock, it would be Amazon.