January ranks as the No. 1 month to buy stocks. But it's in a 12-way tie for first place with all of the other months.

My point, of course, is that anytime is a great time to invest. Starting off the year by choosing stocks with tremendous long-term growth prospects does feel a little extra special, though. And there are plenty of great stocks to consider. Here are three growth stocks that I think are top picks to buy in January (listed in alphabetical order). 

Magnifying glass on top of hundred dollar bills with stock data and a keyboard in the background

Image source: Getty Images.

1. MongoDB

Mongo -- short for humongous -- is a pretty good way to sum up the potential for MongoDB (NASDAQ:MDB). The global database market is growing rapidly -- IDC projects over 50% growth in just the next four years. Companies increasingly want to store data in the cloud. And much of that data is unstructured, like images and videos. That's the perfect storm for MongoDB.

The company developed a database designed to support unstructured data, as well as structured data, that can be run on-premises or in the cloud. MongoDB uses a "freemium" model where customers can use a limited version of its product for free then migrate to the commercial version to get more features and support. This model has worked really well for the company: MongoDB's revenue has skyrocketed more than fivefold over the last three years.

Sure, MongoDB has plenty of competition. Big database companies with deep pockets such as Oracle don't want to give up their share of the market. However, MongoDB is growing much faster than its well-heeled rivals, in part because its technology was specifically built from the ground up for the database world of today while their technology wasn't.

There are several catalysts that should drive MongoDB stock higher in 2020. The most important is the company's Atlas database-as-a-service product. Revenue from Atlas soared 185% year over in MongoDB's fiscal third quarter and should continue to enjoy great momentum. 

2. The Trade Desk

The Trade Desk (NASDAQ:TTD) ranks as a leader in a revolution that many investors haven't even heard about. This revolution is the shift from a cumbersome manual process for buying ads to a programmatic advertising model where software is used to buy ads. The Trade Desk's platform has won multiple awards as the best technology available for buying digital ads programmatically.

Most of ad buying and selling still isn't done programmatically, though. That gives The Trade Desk a lot of room to run, especially considering that programmatic advertising is growing five times faster than the overall ad market.  

The explosion in connected television (CTV) is also dramatically expanding the number of available digital ad opportunities. Streaming services such as Hulu that use ad-supported models have increased the appeal for The Trade Desk's technology. And with the potential for other streaming services to offer ad-supported models to reduce costs for consumers, CTV will likely fuel The Trade Desk's growth even more in the future.

Needham picked The Trade Desk as its top stock to buy for 2020. Analyst Laura Martin likes the same growth drivers that I like. She also highlighted the opportunity for the company to benefit from the U.S. election cycle, with an estimated $10 billion in political ad spending projected this year.

3. Vertex Pharmaceuticals

It's no exaggeration to state that Vertex Pharmaceuticals (NASDAQ:VRTX) has single-handedly changed the dynamics for treating cystic fibrosis (CF). There are only four drugs approved in the U.S. that address the underlying genetic cause of CF. Vertex markets all of them.

This CF franchise has enabled Vertex to grow rapidly over the last several years, but the biotech's growth story isn't over by any means. Vertex's latest CF drug, Trikafta, won U.S. approval in October 2019. Assuming it also obtains approval in Europe, which seems like a good bet, the drug could expand Vertex's addressable patient population by more than 50%.

Vertex's sales will almost certainly soar thanks to Trikafta and to securing key reimbursement deals across the world for its other CF drugs. The company also has its targets set on other rare diseases. Vertex and partner CRISPR Therapeutics, for example, are evaluating a gene-editing therapy for treating rare blood diseases beta-thalassemia and sickle cell disease in early stage clinical studies. In addition to this rare-disease focus, Vertex's pipeline includes promising drugs for alleviating pain. 

My view is that Vertex remains the best biotech stock on the market overall. It claims a monopoly in CF that isn't likely to be challenged anytime soon. It's highly profitable with a growing cash stockpile. It has a newly approved drug that will greatly expand its market opportunity. And it has a pipeline that holds the potential to give Vertex blockbuster drugs in additional indications beyond CF. All of this makes Vertex a great stock to buy in January along with MongoDB and The Trade Desk.