Kurt Goedel
A legendary logician with his outstanding works during the1930's. He was born in Bruenn (Brno), Moravia, and studied in Vienna. He participated in the Vienna Circle, because his principal teacher was Hans Hahn, one of the active members of the Circle; however, his own philosophy was not quite sympathetic to that of the Circle.
He has shown that first order logic is complete (all logical truths are provable within a suitable axiomatic system), and that axiomatic number theory is incomplete (for any axiomatic system for number theory, there are some arithmetic truths unprovable in it). He has also proved an epoch-making theorem on axiomatic set theory.
He spent the latter half of of his life in the Institute for Advanced Study, Princeton; but he spent much time on esoteric philosophy, communicated with very few people, became neurotic and died by refusing to eat. Still, he is one of the best logicians in the 20th century.