Choices, choices. 

The last time I checked, there were around 3,600 stocks listed on U.S. exchanges. And that doesn't include stocks that trade over the counter. 

The more choices I have, the harder it is to make a decision. If you're like me, you know that picking one stock to buy out of thousands is extraordinarily difficult. But despite the difficult challenge, I have made my selection. If I could buy only one stock, it would be Alphabet (GOOG -1.65%) (GOOGL -1.60%). And The Trade Desk (TTD -2.95%). And Markel (MKL -1.94%).

Confused? Allow me to explain.

Man with hand on cheek in front of drawings of light bulbs with one of them colored yellow

Image source: Getty Images.

Framing the question

In Douglas Adams' great book The Hitchhiker's Guide to the Galaxy, a supercomputer named Deep Thought is asked what the answer to the ultimate question of life, the universe, and everything is. After 7.5 million years of computing, Deep Thought's answer was...42. When challenged about the seemingly irrelevant answer, Deep Thought replied that the problem was that no one knew what the ultimate question really was.

Framing the question correctly is important. How a question is presented greatly influences what answer is given. And that's why I listed three different stocks as the "one stock" that I would buy right now.

My three answers stemmed from looking at the question of which one stock I'd buy right now in three different ways. One angle to the question is: Which one stock is my favorite stock? Another is: Which one stock would I buy that I don't already own? Then there's the super-literal interpretation of the question: What stock would I buy if I could only buy one stock and not own any other stocks at all? I answer each of those twists on the question in a different way.

Favorite stock

Choosing my favorite stock to buy right now actually is pretty easy for me. Alphabet is my top stock to buy in 2019, hands down. I've owned shares of the Google parent for several years. If I didn't already own Alphabet, it would be the first stock that I would buy. 

One major reason I like Alphabet is that the company has a powerful moat. By that, I mean that it would be very difficult for a competitor to take away significant market share from Alphabet. Would you want to attempt to launch a product to compete against Google Search, YouTube, and Alphabet's six other products that claim at least 1 billion users per month? Most rational people would answer "no."

I'm also impressed by Alphabet's growth prospects. The company has multiple avenues for growth, and the most important of them is probably its Waymo self-driving-car subsidiary. One analyst has crunched the numbers and projected that Waymo could generate well over $100 billion in revenue by 2030. If this estimate is anywhere close to being right (and I suspect it is), Alphabet is on the path to incredible growth. 

One additional stock

But if you asked me which stock I'd buy right now that I don't already own, my answer would be The Trade Desk. This stock flew under my radar for a while. Thanks to several of my colleagues at The Motley Fool, though, I'm more familiar with it now. And the more I've learned about The Trade Desk, the more I've liked this stock.

The Trade Desk provides a self-service platform that enables ad buyers to purchase and manage digital advertising campaigns. Probably the first thing you think of with digital advertising is the internet. That's certainly a big part of The Trade Desk's fast-growing business. 

Three opportunities should especially boost The Trade Desk's growth in the future. Increased use of mobile advertising is one of them. Expanding into additional international markets is another. But the one that is most intriguing to me is the connected TV (CTV) boom. The Trade Desk CEO Jeff Green said in the company's Q3 conference call that CTV is "the biggest opportunity we've ever seen and probably ever will."

Only one stock

However, if I could buy only one stock and not own any other stocks, it would probably be Markel. The company is like a smaller version of Berkshire Hathaway (BRK.A -2.04%) (BRK.B -1.96%)

Owning Markel provides the benefit of diversification, just as owning an exchange-traded fund (ETF) does. Like Berkshire, Markel is a financial holding company that owns positions in lots of other businesses -- well over 100 of them -- in multiple industries. 

So why pick Markel over Berkshire? Unlike Berkshire, Markel hasn't been slow to buy stocks of strong tech companies, including Alphabet, by the way. Buying Markel also gives you a solid stake in Berkshire because it's Markel's top holding. And there's one other minor detail: Markel has outperformed Berkshire over the last five years.

Check all that apply

If you remember your school days, you'll probably recall that there were several kinds of questions on tests. There was the dreaded essay question. Some questions were binary -- true or false. Others were multiple-choice questions, where you picked one answer from several options. And there was also a special kind of multiple-choice question where you could check all of the answers that applied.

The good news is that in investing, the question is always the check-all-that-apply type. You don't have to limit yourself to buying just one stock. But if you're looking for only one stock to buy right now, Alphabet, The Trade Desk, and Markel appear to be great choices.

Check out the latest Alphabet, The Trade Desk, and Markel earnings call transcripts.