Making the final decision to invest in a given stock can be nerve-wracking. That's especially the case when uncertainty exists about whether or not the positive momentum for the stock market will continue.

The good news is that some of these decisions don't come with nearly as much stress as others. Here are three monster stocks to buy without any hesitation.

1. Amazon

Amazon (AMZN 3.43%) ranks as one of the biggest monster stocks on the planet with a market cap of close to $1.4 trillion. Only three companies that trade on U.S. stock exchanges are bigger. The company is also on a roll, with its shares soaring more than 60% year to date.

Amazon has plenty of room to run. So do many Wall Street analysts. The consensus 12-month price target for the stock reflects an upside potential of around 25%.

The company's profitability continues to improve, thanks to the company's streamlining operations. It's encouraging to see management focused on the bottom line more strongly than in the past.

The biggest tailwind for Amazon, though, is artificial intelligence (AI). I expect AI to provide a huge catalyst for the company's Amazon Web Services (AWS) cloud unit. The potential growth for AWS is reason enough on its own to buy this stock without any hemming or hawing.

2. Mastercard

Mastercard (MA 0.07%) is another mega-cap stock that you can buy without losing any sleep. Its shares have performed well so far this year but aren't at risk of overheating.

Business is booming for the financial services giant. It reported that net revenue in the second quarter jumped 14% year over year to $6.3 billion. Adjusted earnings per share rose 13%.

This growth should continue. Mastercard CEO Michael Mieback noted in the company's Q2 call that, "the overall labor market remains strong, including wage growth, and consumers continue to be supported by credit and savings." With inflation also moderating somewhat, the market dynamics appear to be in the company's favor.

The most compelling argument for buying Mastercard is its moat. The company enjoys a duopoly (along with Visa) in the credit card processing market. With the ongoing shift to a largely cashless society, Mastercard should be able to keep up its winning ways.

3. Vertex Pharmaceuticals

You can throw hesitation out the window completely when it comes to investing in Vertex Pharmaceuticals (VRTX -0.06%). The biotech stock's 31% gain in 2022 and the 19% year-to-date rise could be just a taste of what's on the way.

Vertex is the only drugmaker with approved therapies that treat the underlying cause of rare genetic disease cystic fibrosis (CF). Its CF franchise generated nearly $2.5 billion in revenue and earnings of $916 million in Q2, up 14% and 13%, respectively, year over year.

Any competition in CF is years away, at the earliest. In the meantime, Vertex continues to make advances in the indication. The company expects to report results from a late-stage study of what could be its most powerful and most profitable CF therapy yet in early 2024.

But Vertex's biggest opportunities lie outside of CF. The drugmaker is awaiting regulatory approvals for CRISPR gene-editing therapy exa-cel in curing rare blood disorders sickle cell disease and transfusion-dependent beta-thalassemia. It has a promising non-opioid pain drug, VX-548, that should be on the regulatory approval path next year.

In addition, Vertex is evaluating inaxaplin in a phase 2b study targeting APOL1-mediated kidney disease. The genetic disorder affects more patients globally than CF does. Vertex also has a promising program in clinical testing that holds the potential to cure type 1 diabetes.