Claudius Ptolemaeus, no contest, but that was a much more laid
back millenium. There may be a credible Chinese entry as well,
but I can't recall him (or her). Ptolemy invented laboratory
science as we know it in addition to propounding the most
successful cosmology ever measured in terms of its duration of
acceptance by other scientists.
There are several contenders in the fourth millenium, the one
before Ptolemy's (we've just finished the sixth millenium). My
vote would probably go to Pythagoras or Archimedes.
Can anyone name a third millenium scientist?
Incidently, my proposal of Josiah Willard Gibbs for the sixth
millenium award is purely chauvinistic, but he's a contender!