Kepler was an astronomer. He worked as an assistant to Brahe. Kepler used Brahe's observations to prove that planetary orbits were ellipses.
Einstein was a mathematician and a physicist. He was not an astronomer. He used mathematics to develop his theory of Relativity.
Therefore, it you specifically ask for an "astronomer", we have to go with Kepler.
Tycho Brahe was THE MOS PRECISE astronomer of the pre-telescopic era. Where other astronomers would observe a planet two or three times in a night, Tycho would observe it a hundred times, noting the specific observed position every time AS IF it were a separate observation.
THEN, he would apply statistical analysis to the sum of all his observations, so that his accuracy was ten times better than the others (ten = square root of a hundred).
Without Tycho's observations, Kepler would not have been able to make his history-changing calculations.