It's hard for me to pick my top favorite in many categories. Whether talking about actors, athletes, songs, singers, movies, or TV shows, I find it difficult to single one out as the best.

This challenge applies to stocks as well. There are quite a few stocks I think will be big long-term winners. What if I had to choose just one to invest $1,000 in right now? It's not an easy decision, but here's why Vertex Pharmaceuticals (VRTX -0.06%) is my pick as the best stock.

A safe haven if the stock market falls

Warren Buffett once said that rule No. 1 in investing is to "never lose money." He added that the second rule is "don't forget the first rule." Of course, no stock is guaranteed never to lose you money. However, I think Vertex is the kind of safe haven stock that protects you more than most if the stock market falls.

We saw this happen in 2022. The S&P 500 fell nearly 20% that year. The Nasdaq Composite Index plunged 33%. Vertex's share price, though, soared almost 32%.

Many investors realize that Vertex's business is remarkably resilient. The company enjoys a monopoly in treating the underlying cause of cystic fibrosis (CF). There's practically zero risk that physicians will quit prescribing Vertex's CF therapies during an economic decline or a stock market crash.

Vertex is highly profitable, raking in earnings of $2.65 billion during the first nine months of 2023. It also sits atop a hefty cash stockpile of close to $13.5 billion (including cash, cash equivalents, and marketable securities).

Multiple growth drivers

This wouldn't be the best stock to invest $1,000 in right now, though, if it were only a defensive play. Vertex also has multiple growth drivers.

First, the company should be able to continue growing in the CF market. It has opportunities to treat younger patients with its current drugs. Vertex could soon file for regulatory approvals of what I expect to be its most powerful and most profitable CF therapy yet -- a vanzacaftor triple-drug combo. It's working with Moderna to develop a messenger RNA therapy for treating patients who don't benefit from current drugs.

Second, Vertex recently won U.S. approvals for Casgevy in treating sickle cell disease and transfusion-dependent beta-thalassemia. It also picked up U.K. approvals as a treatment for both rare blood disorders. The gene-editing therapy could reach peak annual sales of $3.9 billion, according to Goldman Sachs.

Third, the company has two other promising drugs in pivotal clinical testing. Vertex hopes to move forward soon with regulatory filings for VX-548 in treating acute pain. The drug doesn't have the side effects and addictive qualities of opioids. The big biotech is also advancing inaxaplin into a late-stage clinical trial in the first quarter of 2024. The drug targets APOL1-mediated kidney disease, an indication that affects more patients than CF and has no approved therapy that treats the disease's underlying cause.

While those are Vertex's major potential growth drivers for now, it could have others in the not-too-distant future. The company is evaluating several programs that could cure type 1 diabetes in early-stage testing as well as an experimental drug targeting myotonic dystrophy type 1, a type of muscular dystrophy.

An attractive valuation to boot

I ruled out some stocks I like from being the best to buy now because of their valuations. That's not a problem with Vertex, though.

Granted, the biotech stock might appear expensive at first glance, with a forward price-to-earnings ratio of nearly 27. However, Vertex looks quite cheap with its growth prospects factored in. Its price-to-earnings-to-growth (PEG) ratio is a super low 0.6.

The main risk with Vertex is that those growth estimates don't materialize. However, I think the company's late-stage pipeline programs are more likely to succeed than fail. Investing $1,000 in the stock could pay off handsomely over the next few years.