What's the least common element in the universe? And what is its frequency measured in Hertz?
(edited based on @JonCuster's comment) The rarest known elements are most probably the man-made heavy elements. These isotopes (elements) are very unstable. They most probably also formed in supernovas, but their natural formation is minimal, and due to their short lifetime, they do not accumulate either.
As Jon added, most probably heavier elements are also formed in supernovas, however, I am not aware of any of them observed. The reason for that is 1) again, short lifetime prevents any large accumulation, 2) under the formation conditions they are not atoms, but bare nuclei /plasma, so one cannot observe their atomic spectrum.