The American pharmaceutical giant Pfizer and Irish counterpart Allergan announced a plan to merge late in November. The planned merger would be the second-largest M&A deal on record behind Vodafone AirTouch's $172.0 billion acquisition of Mannesman in 1999, according to Dealogic.
The terms of the deal will allow Pfizer to domicile in Ireland, where corporation tax is just 12.5 percent, significantly less than the 40 percent rate charged in the U.S.
Among Pfizer's advisers was Goldman Sachs, which topped worldwide rankings by advising on 408 deals worth a total of $1.7 trillion in 2015.
2. AB InBev and SABMiller: $120 billion
The Belgian brewer Anheuser-Busch InBevreached an agreement to buy U.K. rivalSABMiller in mid-November.
Combining the world's two biggest brewers will form a company that makes almost a third of the world's beer supply.
Among the advisors to Anheuser-Busch InBev were Lazard, Deutsche Bank and Barclays.
3. Shell and BG Group: $81 billion
Royal Dutch Shell announced plans to acquire British energy supplier BG in April. The rationale for the merger was to diversify Shell's operations by improving its liquefied gas and deep water exploration capabilities.
Bank of America Merrill Lynch was the sole adviser for the Anglo-Dutch company, according to Thomson Reuters. Over the course of 2015, the bank was the fourth most active adviser, working on 240 deals worth $1.0 trillion.
4. Charter and Time Warner Cable: $78 billion
Late in May, U.S. broadband provider Charter Communications announced plans to merge with Time Warner Cable.
The deal, valued at $79 billion, vastly increased Charter's customer base by expanding its U.S. footprint by 84,000 square miles. Among Charter's advisors were Goldman Sachs, Bank of America Merrill Lynch and Credit Suisse.
5. Dow Chemical and DuPont: $68 billion
The science and agriculture companies announced a "merger of equals" in December, aimed at producing a giant with a market capitalization of $130 billion and cost-synergy savings of $3 billion.
However, within two years of the completed merger, the newly formed DowDuPont intends to break up into three companies focused on agriculture, material science and technology respectively, according to the press release from DuPont.
Klein and Company, Lazard and Morgan Stanley advised Dow Chemical on the deal, while Evercore and Goldman Sachs assisted DuPont.
6. Dell and EMC: $66 billion
Dell signed an agreement to acquire data storage company EMC in October. The deal was described as the second-biggest technology merger ever by Fortune, behind the $106 billion tie-up between AOl and Time Warner in 2000.
"Dell and EMC are a dream combination," Michael Dell, founder and CEO of the tech giant, said in a public speech. "We complement each other beautifully."
Among Dell's advisers were JPMorgan, Credit Suisse and Barclays.
7. Energy Transfer Equity and Williams: $56 billion
Energy Transfer Equity spent a year pursuing energy company Williams. An initial offer of $48 billion in June was rejected, but a deal was struck by late September.
The merger will create the world's largest energy infrastructure group, although the Federal Trade Commission must first review the deal.
Goldman Sachs, UBS and Citi were among Energy Transfer Equity's advisers.
Citi worked on 275 M&A deals worldwide in 2015, which were worth a total of $811 billion.
8. Heinz and Kraft Foods: $55 billion
Heinz and Kraft completed their merger early in July.
The newly created Kraft Heinz Companybecame the third-largest food company in the U.S. and the fifth-largest in the world by annual sales.
Heinz's adviser in the acquisition, Lazard, worked on 245 deals in 2015 worth a total of $635 billion.
9. Anthem and Cigna: $48 billion
The proposed merger between Anthem Incand Cigna came under a lot of scrutiny after it was announced late in July. The deal would create the largest health insurer in the U.S., with around 53 million customers and there were concerns from the American Medical Association about reduced competition.
In December, shareholders of both companies voted in favor of the $48 billion deal. The companies expect the merger to complete later in 2016, once regulators have finished reviewing the deal.
UBS and Credit Suisse assisted Anthem on the buyout. Credit Suisse worked on 215 deals through 2015 worth a total of $718 billion.
10. EBay shareholders and Paypal: $47billion
In June, e-commerce giant eBay announced it would spin-off online payment providerPaypal, which it had acquired in 2002. As each eBay shareholder received one Paypal stock for each eBay stock, Thomson Reuters have listed the spin-off as the 10th biggest M&A deal of the year.
In a press release, eBay CEO John Donahoe said the rationale for splitting the company was to make each business more competitive in order to meet future challenges.
"As independent companies, eBay and PayPal will enjoy added flexibility to pursue new market and partnership opportunities," he said.
When Paypal started trading on the Nasdaq in July, it actually had a larger market capitalization than its parent company.