Isolation of the metastable isotope technetium-99m, the most commonly used medical radioistope, used in tens of millions of medical diagnostic procedures annually. Segrè discovered the first artifical element Technetium in December 1936.

"This letter was received for publication on the date indicated (January 28, 1941), but was voluntarily withheld from publication until the end of the war."
Seaborg and McMillan discovered element 94, which they named plutonium. In 1951 Seaborg and McMillan shared the Nobel Prize in Chemistry "for their discoveries in the chemistry of the transuranium elements."

Order of authorship in the original publication: Seaborg, McMillan, Kennedy, Wahl.

(Thanks to Juan Weiss for this reference and its interpretation.)