Microsoft-Nokia ($7.2B, 2014)

Microsoft's $7.2 billion purchase of Nokia's phone division in 2014 was immediately written off within a year, resulting in 18,000 employee layoffs. The deal aimed to challenge iPhone and Android but delivered no sustainable mobile market share, making it Steve Ballmer's most expensive single mistake. Financially, it was a 100% loss in value — a total wipeout that even outperforms the 80% write-down HP suffered on Autonomy (#3) in severity.