An amazing mathematician, to put it lightly. I remember learning the Atiyah-Singer index theorem [1] and how it relates to anomalies in quantum theories, it absolutely blew my mind. And then every time I came back to anomalies I would rediscover this fact and be floored again, one of my favorite physics/theory/math connections.

[1] e.g. https://en.wikipedia.org/wiki/Atiyah%E2%80%93Singer_index_th... (though not the best for the more physics minded)