Theranos

Elizabeth Holmes convinced Walgreens, Safeway, the U.S. military, and investors including Rupert Murdoch to pour $700 million into blood-testing technology that didn't work. At its peak, Theranos was valued at $9 billion. The Edison machine couldn't reliably perform a single one of the 200+ tests Holmes claimed it could. She's now serving 11 years in federal prison. It's not just the biggest tech flop — it's the biggest fraud in Silicon Valley history.