He developed quantum electrodynamics, the first fully fleshed-out quantum field theory. In the process, the invented the action formulation of quantum field the

He developed quantum electrodynamics, the first fully fleshed-out quantum field theory. In the process, the invented the action formulation of quantum field theory, which is absolutely fundamental to the modern understanding of the subject, and he invented the method of solving path integrals perturbatively that everyone has used since (Feynman diagrams). That easily puts him among the top 10 physicists of the 20th Century. Beyond his research contributions, he was a highly innovative und unorthodox teacher, and an utterly captivating raconteur. He had a highly unusual combination of skills and personality traits. That's why he's so famous.