This fellow invented relativistic perturbation theory in 1934, almost 2 decades before Feynman and Schwinger. He discovered that positrons are back-in-time electrons in 1938 (and Feynman got the idea through Wheeler indirectly from him). Stueckelberg proposed renormalizable electrodynamics in 1941 but his paper was rejected from physical review, it took Hans Bethe's 1947 Lamb-Shift estimate to reintroduce the subject. Stueckelberg should have received the 1965 Nobel prize along with Schwinger and Feynman, while Tomonaga could have shared his with Luttinger for 1d liquids (which is a bigger contribution of Tomonaga's anyway). Stueckelberg didn't rest on his laurels, he went on to discover the Abelian Higgs mechanism and the renormalization group too. Each of these are major discoveries in themselves, but to have one person discover all of them raises him to Bohr-Einstein status. He died insane and neglected, although he received some awards late in life. He is the godfather of underrecognized physicists, and he must be at the top of any list.
>Why was he underrecognized?
He was antisocial.