AlphaGo

First AI to defeat a human professional Go player. Combined deep neural networks with Monte Carlo tree search. Defeated European champion Fan Hui 5-0 (October 2015), then world champion Lee Sedol 4-1 in March 2016, watched by 200M+ viewers.

AlphaGo was a watershed moment for AI, demonstrating that deep learning could master a game long considered intractable for computers. Its successor AlphaZero (2017) generalized to chess and shogi without human knowledge. Nature 2016. By Silver, Huang et al. (DeepMind).