Bayer-Monsanto ($63 Billion, 2018)

Bayer's $63 billion acquisition of Monsanto turned into a legal and financial nightmare, as the company inherited over $10 billion in Roundup cancer lawsuits. By 2020, Bayer had lost more than its entire purchase price in market capitalization, falling 45% below pre-deal levels. This is the only merger on this list where litigation costs exceeded the acquisition price, making it far riskier than the $26 billion Sprint-T-Mobile deal, which faced antitrust challenges but no product liability exposure.