Tough to pick against either of these guys. Einstein's breakthroughs have been pushing physics to exciting new places for nearly a century now, and he made wild hair seem like a terrific cosmetic decision. Salk, though, saved us from polio, and by establishing the Salk Institute for Biological Studies in La Jolla, California, he created a place where top-flight scientific research is still being done almost 14 years after his death. Plus, he was popular with the ladies; Salk's second wife was Francoise Gilot, a former mistress of Picasso. Einstein may be thought of as the 20th century's greatest genius, but it's tough to diminish Salk's brilliance. Who's going to take your vote here, the biologist or the physicist?
[See the whole bracket here.]