Dec. 28 was the date the Nasdaq Composite Index reached a bottom after declining throughout much of the previous 12 months.

To kick off a new bull market, the index will need to rise by at least 20% above that low. At the end of March, the Nasdaq needed to climb less than 1% to reach that threshold. It's quite possible that a Nasdaq bull market could begin in April. With that in mind, here are three top stocks to buy now.

1. Alphabet

Like the Nasdaq Composite Index, Alphabet (GOOG 0.76%) (GOOGL 0.79%) is getting really close to entering bull market territory. Shares of the tech giant climbed more than 20% above its previous low a few days ago before giving up some of its gains.

Alphabet continues to face headwinds in the advertising market related to overall economic uncertainty. However, these issues should only be temporary. Some investors have worried that the company has a bigger threat on its hands with competition from OpenAI's ChatGPT and Microsoft's integration of the chatbot with its Bing search engine. So far, though, those fears appear to be overblown.

Over the long term, Alphabet seems likely to continue its dominance in search. The company will almost certainly to rank among the leaders in artificial intelligence. It also has a potentially massive opportunity in quantum computing down the road.

Meanwhile, Alphabet's valuation remains near its most attractive level in years. It's also the cheapest FAANG stock based on several valuation metrics. When the next Nasdaq bull market starts, expect Alphabet to be a big winner.

2. The Trade Desk

The Trade Desk (TTD 0.61%) is already in a roaring bull market of its own. Shares of the digital advertising platform company have skyrocketed more than 50% from its bottom reached in early November 2022.

The company's business has proven to be exceptionally resilient in a tough advertising market. Ad spending on its platform has grown three times faster than the industry average. One key reason behind The Trade Desk's outperformance is that advertisers are trying to maximize their return on investment with their marketing budgets. The Trade Desk's platform enables them to achieve that goal.

Another important tailwind for the company is the explosive growth in ad-supported connected TV (CTV). The Trade Desk CEO Jeff Green has said several times that "CTV is changing everything in advertising."

The biggest knock against the adtech stock is its valuation. Shares currently trade at well over 50 times forward earnings. However, with its huge growth prospects, The Trade Desk should have plenty of room to run.

3. Vertex Pharmaceuticals

Vertex Pharmaceuticals (VRTX 0.43%) delivered a tremendous gain of nearly 32% last year while the broader market sank. The biotech stock is trailing the Nasdaq Composite Index so far this year but is still performing well.

Good news could be on the way for Vertex. The company and its partner, CRISPR Therapeutics, expect to win regulatory approvals for exa-cel in the near future. The gene-editing therapy should be a big commercial success if approved in treating sickle cell disease and transfusion-dependent beta-thalassemia.

Vertex's pipeline also features two other programs that could be on track to launch within the next couple of years. The company is evaluating non-opioid pain drug VX-548 and a triple-drug combo treating cystic fibrosis in late-stage studies. Both could generate peak sales of more than $2 billion if they're approved.

In the meantime, Vertex continues to generate impressive profits with its current cystic fibrosis drugs. This franchise has enabled the company to build a big cash stockpile that it can use (and is using) to invest in further expanding its pipeline. Vertex is the kind of stock that should flourish whether or not a new bull market begins.