Bill Ackman was among the best performing hedge fund managers of 2014. Pershing Square International, one of his four hedge funds, returned more than 40% last year, according to Bloomberg. Given his stellar track record, investors may be interested in keeping an eye on his holdings. Let's take a look at some of Ackman's largest positions.

His largest holding may be an exception
As of the end of March, Ackman owned roughly 19.5 million shares of Valeant Pharmaceuticals (NYSE:BHC), making it his fund's largest holding. In total, Pershing Square owns about 5% of the company.

Normally, this would come across as an unfortunate turn of events for Valeant CEO, Michael Pearson. When Ackman initiates a stake in a firm, it's often under the pretense of forcing a change in management. But the stake in Valeant seems different.

Pershing Square worked closely with Valeant in 2014, partnering with the firm to try to force the acquisition of Botox-maker Actavis. The deal never materialized, but Pershing Square may have discovered something it likes. Reuters reports that Pershing Square intends to take a passive approach toward Valeant. And, thus far at least, that appears to be the case. Since the hedge fund disclosed its Valeant stake earlier this year, it has made no public attempts to shake things up at the pharmaceutical giant.

Coming up on the one year anniversary
Pershing Square's second largest holding is Air Products & Chemicals (NYSE:APD), a more traditional Ackman investment. Pershing Square began amassing its Air Products stake in the summer of 2013, with Ackman pushing for change shortly thereafter. Management caved, and Air Products' CEO announced his retirement. Last June, the firm named his replacement.

Since then, Air Products has outperformed the broader S&P 500 -- though, at $145 a share, the stock remains well below Ackman's $200 price target. Earnings have grown, but sales have lagged. Last quarter, Air Products' net income rose 17% on an annual basis, but sales rose only 1%. Pershing Square, however, could remain in the stock for quite some time, as Ackman originally offered up a three-year time horizon.

One of his biggest winners remains one of his largest holdings
Ackman has had a number of notable wins over the years, but Canadian Pacific (NYSE:CP) stands out as one of the best. More than three years after he won an intense proxy battle, Canadian Pacific remains Pershing Square's third-largest holding. Since Hunter Harrison, Ackman's handpicked CEO, took command of the company in June 2012, shares have risen nearly 130%.

Ackman remains a Canadian Pacific director, though Pershing Square has begun to slowly unload its position. While Pershing Square once held more than 24 million shares, the stake has been reduced to just under 14 million. 2014 was a strong year for the Canadian railroad giant, with a string of solid quarterly earnings fueled by successful cost-cutting measures. In October, Canadian Pacific reported operating income of $621 million, a 19% annual increase and its highest ever. .

A possible sale?
Pershing Square's fourth-largest holding is another pharmaceutical company, the veterinarian drugmaker Zoetis (NYSE:ZTS). Zoetis is a relatively new stake for Pershing Square -- it acquired the stock late last year, and, to date, it's done little to actively change the company.

In February, Zoetis announced that it would appoint one of Ackman's representatives to its board of directors. That representative, Bill Doyle, was instrumental in Pershing Square's Actavis trade, and he could push for a sale of Zoetis, which was once a division of drug giant Pfizer. Ackman has since added a second member to Zoetis' board, but hasn't publicly disclosed his plans for the company. In a letter to investors, he wrote that Zoetis was an attractive business, with a strong model that, in contrast to other firms in the space, didn't rely on patent-protected drugs to succeed.

Betting on a collapse
Finally, I would be remiss if I failed to mention Herbalife (NYSE:HLF), the controversial multi-level marketer. Herbalife doesn't show up in Pershing Square's SEC filings -- as a short position it doesn't need to be disclosed -- but Ackman hasn't shied away from publicity in his quest to take the company down.

Ackman believes that Herbalife is running an illegal pyramid scheme, defrauding millions of consumers with a complicated arrangement that fosters artificial demand of Herbalife's overpriced commodity products. The billionaire hedge fund manager's prodding has been enough to prompt the interest of regulators. The Federal Trade Commission is currently investigating the company, and if it determines that Ackman's claims have merit, it could shutter the company and wipe out shareholders. Were that to occur, Pershing Square could make upwards of $1 billion.

Initially, Ackman disclosed that he was short about $1 billion worth of Herbalife common stock; but he has since reconfigured his position using put options. After his first presentation on Herbalife at the end of 2012, several other hedge funds -- including those run by famed investors Carl Icahn and Dan Loeb -- went the other direction and purchased large stakes in Herbalife. That sent shares surging higher, as investors began to doubt the validity of Ackman's claims (a turn of events which Ackman, by his own admission, did not foresee). Given the risky nature of outright shorting -- potential losses are theoretically unlimited -- Ackman subsequently covered 40% of his short and purchased put options on Herbalife instead.

In a letter to his investors in October 2013, Ackman explained his decision to reconfigure Pershing Square's Herbalife stake. The put contracts should net him a similar gain in the event of Herbalife's collapse while simultaneously limiting his potential downside. Unfortunately, he hasn't broken down the exact nature of the put option portfolio, but he has admitted to holding put options dated January 2015. Ahead of their expiration, Ackman rolled them over at least once. .

For its part, Herbalife disputes Ackman's claims entirely. Herbalife's earnings reports have been mixed over the last two years, but if the FTC were to exonerate the company, shares would likely surge as the prospect of a shutdown would vanish.