In 1934 he devised a fully covariant perturbation theory for quantum fields. To quote this paper, "The approach proposed by Stueckelberg was far more powerful, but was not adopted by others at the time". Now, despite its benefits, this approach has been all but forgotten. However, besides being explicitly covariant, Stueckelberg's methods avoid vacuum bubbles. See also here.
Independently from Hideki Yukawa, he gave vector boson exchange as the theoretical explanation of the strong nuclear force in 1935.
In 1938 he recognized that massive electrodynamics contains a hidden scalar, and formulated an affine version of what would become known as the Abelian Higgs mechanism.
The evolution parameter theory he presented in 1941 and 1942 is the basis for recent work in Relativistic dynamics.
In 1942 he proposed the interpretation of the positron as a negative energy electron traveling backward in time.
Although his work was mostly well respected, later work that was only superficially different and only marginally more advanced would win the Nobel prize for others.
