Ken Griffin founded Citadel in 1990. It's now the most successful hedge fund ever. Through Citadel, Griffin built a personal fortune of $37 billion. Citadel's portfolio is loaded with artificial intelligence (AI) stocks these days, with more than half a dozen of them in its top 50 holdings. There's one, though, that Griffin appears to be especially bullish about. The billionaire has more than tripled his investment in this AI stock. Amazon (NASDAQ: AMZN) ranked as Citadel's fourth-largest holding at the end of 2023.