If you had to pick just one stock to buy today and hold forever, which one would you choose -- and why?

Of course, there is no reason to actually do this in a real-world portfolio. Long-term investors should put together a couple of dozen names or build a smaller collection around a diverse market-index tracker such as the Vanguard S&P 500 ETF.

It's still a fun exercise to identify the top stock to trust for the long haul with no support from other tickers in a variety of market sectors. Very few names can stand on their own like that.

For example, it's no secret that I love Netflix (NFLX -3.92%) and Fiverr International (FVRR 1.34%), and I would recommend them to anybody right now. But I would never bet my entire portfolio on Netflix dominating the entertainment industry or Fiverr reshaping the world of work. Many things could go wrong along the way and wreck my only investment. They are not cut out to carry all of my life savings forever.

I would be more interested in multi-sector conglomerates such as Berkshire Hathaway (BRK.A -0.28%) (BRK.B -0.68%) or Amazon (AMZN -1.64%). These companies have established themselves as leaders across several industries, centered on Berkshire's insurance offerings and Amazon's e-commerce service. They are diversified enough to glide through pretty much any economy and market environment, dodging the punches in one market by doubling down on a different opportunity.

But I still wouldn't take Amazon or Berkshire Hathaway to a desert island. As nimble as they are, I don't think there's a company on the planet that can match the long-term flexibility of Alphabet (GOOG 0.74%) (GOOGL 0.55%).

Alphabet is so much more than Google and Android

Most people look at Alphabet and see nothing but an online service portal. That's the parent company of the Google search engine and the Android smartphone brand, and you might recall that it owns the YouTube video portal, too. All of this is true, of course, and services under the Google umbrella currently account for more than 99% of Alphabet's quarterly revenue.

From this point of view, you might think that I should have picked Amazon or Berkshire instead. Their business models are already diverse, while Alphabet seems to be tightly focused on a single opportunity.

But nothing could be further from the truth. Alphabet is exploring myriad business opportunities, and many of them may dwarf the Google name in the long run.

Many side bets

The company formerly known as Google reorganized under the Alphabet umbrella in 2015. It was time to separate the company's sprawling ambitions from the single-sector Google name. This way, Alphabet should be able to develop and monetize wildly different business operations under separate brands without confusing consumers, investors, and regulators. Many years down the road, this organization may also make it easier to spin off fully mature businesses as independent companies.

Early examples of non-Google ideas include the Waymo brand of self-driving vehicles, the Calico health research branch, and the CapitalG technology investment firm. Other projects are exploring high-speed internet services for rural areas, automated drug discovery platforms, robotics software, and much more.

These alternative ideas can tap into Alphabet's powerful artificial intelligence tools, which in turn were built on data collected in the search and advertising business. This is a unique business advantage that sets Alphabet up for decades of market-beating success.

Game-changing ideas

Some of Alphabet's side bets don't look like game-changers. Internet access is a fairly mature market already, and one more competitor won't set the world on fire.

However, self-driving cars could absolutely become a global business with massive revenue streams. The same is true for drug development, artificial intelligence services, and robotic control systems. That's four potential world-breakers, and we're just scratching the surface of Alphabet's wide array of target markets. Plus, we probably haven't even heard of many future billion-dollar business ideas on Alphabet's bench. Not yet.

This company is not only willing and able to turn on a dime but actively seeking opportunities to do it. Google's online search and advertising will probably run out of steam eventually, and that would spell the end for a less flexible company. But Alphabet will keep going until it runs out of ideas, throwing machine-assisted brain power at a ton of seemingly unrelated problems along the way.

That is how you build a market-leading business for the very long run. I can't imagine Alphabet running out of steam in my lifetime. Not even Berkshire Hathaway or Netflix can live up to that lofty standard. So if I really had to pick just one stock for the rest of my life, Alphabet would be it.

And it doesn't hurt that Alphabet's two stock classes are trading closer to their 52-week lows than to last November's all-time highs. This looks like a great time to pick up the ultra-safe Alphabet stock on the cheap.