What's the single greatest advantage you have as an investor? Time. As long as your time horizon is long enough, you don't have to worry about many of the things that can happen with a stock that can cause angst, such as a disappointing quarterly update.

There's no longer time horizon than forever, although no one can literally buy and hold a stock forever. However, you could buy a stock and never sell it, ultimately passing down ownership to your children.

But are there stocks worthy of holding on to for such a long time? I think so. Here's why Alphabet (GOOG 1.44%) (GOOGL 1.89%), The Walt Disney Company (DIS 0.42%), and Markel (MKL 0.16%) are three stocks that you can keep practically forever.

One hundred dollar bills linked together to form an infinity symbol.

Image source: Getty Images.

1. Alphabet

Alphabet is best known as the parent of Google. Its products such as Google Search, YouTube, Android, Chrome, Gmail, Google Play, and Maps are each used by more than 1 billion people every month.

This huge user base is actually one of the key reasons why Alphabet is a stock that you can keep forever. Several of Alphabet's products enjoy what's known as the network effect. With a network effect, the more people who use a product, the more valuable the product becomes. It's also extremely difficult for a rival to compete against products with strong network effects. That's what has happened for quite a few of Alphabet's products, especially Google Search and YouTube.

Another nice plus for Alphabet is that its popular products generate a ton of cash. In 2018, the company made $136.8 billion in revenue, $30.7 billion in profit, and $19.8 billion in free cash flow. Alphabet now claims an impressive cash stockpile of over $109 billion. That's money the company can use to invest in improving its current products (and increase its network effect), as well as invest in new growth opportunities.

That leads us to another reason why Alphabet is such a great stock -- the company's famous "other bets." Alphabet has invested in multiple innovative technologies that hold tremendous potential. The one that I think is likely to be the biggest winner is the company's Waymo self-driving car subsidiary. UBS analyst Eric Sheridan even projects that Waymo will make more than $100 billion in revenue by 2030, nearly as much as Alphabet's Google business made last year.

2. Disney

There aren't many brands with the enduring appeal that Disney has had through the years. The company is well known for its theme parks, movie studios, and television networks. 

Probably the greatest strength for Disney is its ability to make money from its intellectual property in multiple ways. For example, the company leveraged the success of its Frozen movie to develop a ride for its theme parks and create multiple products based on the movie, including toys and clothing. And, of course, Disney has a sequel on the way that's likely to be another smash hit. This approach to monetizing assets in multiple ways is common for the company.

There are plenty of reasons to like Disney over the near term. Of the 10 most anticipated movies for 2019, seven are Disney productions. The company plans to launch new Star Wars-themed lands in Disneyland and Walt Disney World this year, and also plans to roll out its Disney+ streaming service in 2019.

But the important thing for long-term investors is that Disney has what it takes to continue introducing highly anticipated new products and services for years to come. Success begets success. The blockbuster movies of this year could become a new live show or theme park attraction in a few years down the road. And Disney will keep on repeating its winning formula, just as it has for decades. 

Check out the latest earnings call transcripts for Alphabet, Disney, and Markel.

3. Markel

You might not have heard of Markel. The company's primary business is insurance that provides coverage for specialty markets like classic cars, prize horses, and vintage boats. That might not seem as exciting as Google or Disneyland, but underwriting specialty insurance has made Markel very profitable.

If Markel was nothing but an insurance company, I might not be very enthusiastic about the stock. But it's not just an insurance company. Markel is actually a financial holding company that owns lots of other businesses, similar to Warren Buffett's Berkshire Hathaway. Actually, Markel is sometimes referred to as a "baby Berkshire," with the baby reference stemming from its market cap that's much lower than Berkshire's.

Markel wholly owns 19 businesses that aren't connected to its insurance operations. These include several manufacturers, healthcare services providers, and financial services companies. Markel's latest addition to its portfolio of companies was fashion leather handbag maker Brahmin.

Like Berkshire, Markel also invests in publicly traded companies. The company's investment portfolio includes well over 100 stocks spanning multiple industries, including Alphabet and Disney. Buying Markel is like buying a fund with diversified holdings -- which isn't a bad approach to making solid returns over the long run.

What sets these stocks apart

There are a couple of things that I think set Alphabet, Disney, and Markel apart from most stocks. One is that all three companies have businesses that are likely to endure for a long time to come. In other words, they enjoy economic moats -- sustainable competitive advantages. All three companies also have multiple growth opportunities, also known as optionality.

Alphabet, Disney, and Markel aren't the only stocks that are good candidates to buy and keep forever. But any stock that makes the list should have a moat and optionality like they do.