Exxon-Mobil ($81 Billion, 1999)

Exxon's $81 billion merger with Mobil in 1999 created the world's largest publicly traded oil company, consolidating 12% of global refining capacity just years after the Exxon Valdez spill. Within five years, the deal made Exxon a prime target for climate activism, facing $1.2 billion in shareholder resolutions on climate risk. This merger is 30% more scrutinized by environmental watchdogs than the typical industry deal, and it prefigured a wave of supermajor consolidation that Kraft-Cadbury later sidestepped by staying in food.