Diversification, shmiversification. Billionaire Bill Ackman doesn't subscribe to the theory that he needs to own lots of stocks to be successful.

His Pershing Square Capital Management hedge fund holds positions in a total of only 12 stocks. Nearly one-third of Ackman's $11.9 billion portfolio is invested in three magnificent growth stocks.

Ackman's new top stock

Ackman now has a new top stock in his portfolio. In February, he revealed via a social media post on X (formerly Twitter) that Pershing Square was buying shares of Uber Technologies (UBER -0.16%).

Beginning in early January, we began acquiring a position in @Uber. Today, we own 30.3 million shares. I have been a long-term customer and admirer of Uber beginning when Edward Norton showed me the app in its early days. I was also fortunate to be a day-one investor in the...

-- Bill Ackman (@BillAckman) February 7, 2025

Pershing Square still owned 30.3 million Uber shares worth roughly $2.21 billion at the end of the first quarter of 2025. This stake in the ride-hailing services company made up 18.5% of the hedge fund's portfolio, edging out Brookfield Corp. as its largest holding.

Image source: Getty Images.

Ackman's X post mentioned several reasons he invested in Uber. First, the billionaire said he was "a longtime customer and admirer" of the company. That fits squarely into the philosophy espoused by legendary investor Peter Lynch of buying what you know. Ackman also called Uber "a highly profitable and cash-generative growth machine."

Arguably, its valuation was the most important factor in his decision to buy the stock. Ackman thought Uber traded at a steep discount to its intrinsic value. However, since his social media post, the stock has soared.

A two-for-one special

An additional 14% of Pershing Square's portfolio is invested in another technology giant. But that percentage is split across two shares. Ackman owns both Alphabet Class A shares (GOOGL -1.37%) and Alphabet Class C shares (GOOG -1.34%).

He first invested in the Google parent company in early 2023. Many were skeptical about Google's prospects in the aftermath of OpenAI's launch of ChatGPT. Some even predicted that the generative artificial intelligence (AI) technology popularized by ChatGPT presented an existential threat to Google Search.

Ackman disagreed. He quickly took advantage of Alphabet's sell-off and built a large position in its two stocks. Today, the company's Class A and Class C shares combined make it Pershing Square's third-largest position. That move has paid off nicely despite Alphabet's poor stock performance in 2025.

Interestingly, Ackman has both bought and sold Alphabet recently. He increased his hedge fund's stake in the Class A shares by around 11.3% in Q1 but reduced its position in the Class C shares by 16.2%.

Are these Ackman stocks good picks to buy now?

Not everyone agrees with Ackman about Uber's valuation. The stock trades at nearly 32.6 times forward earnings. Its price-to-earnings-to-growth (PEG) ratio, which is based on analysts' five-year earnings growth estimates, is a sky-high 14.5. AlphaSpread's discounted cash flow (DCF) model puts Uber's intrinsic value at roughly 18% below its current share price.

However, calculating an intrinsic value uses many assumptions. SimplyWallSt estimates that Uber is undervalued by nearly 47%. Ackman would probably agree more with the assumptions used in that analysis than he would with AlphaSpread's. My problem with buying Uber stock right now, though, is that I'm not sure which assumptions are closer to being right.

What about Alphabet? The company faces some uncertainties, especially with the antitrust rulings that have gone against it.

But I like Google Cloud's growth prospects with the rapid adoption of generative AI. I like the potential for Alphabet's self-driving car business, Waymo (which has partnered with Uber, by the way). I also predict that Google Search will continue to integrate with AI to improve its capabilities and hold on to the massive user base that attracts advertising revenue. Overall, I view Alphabet as the best pick in Ackman's portfolio.