Archimedes (287  212 B.C.)
In his Measurement of the Circle Archimedes found an
approximation value for pi by approximating the circumference of
the unit circle by the perimeter of inscribed and cirumscribed
polygons. Using polygons with 96 sides he found
3.1408 < pi < 3.1428
This was just one of Archimedes' many accomplishments. Among
others are

Formulas for the area and volume of the sphere

Formulas for the area of a parablolic segment

Discovery of a principle (specific gravity) by means of
which alloyed gold can be distinguished from the pure
substance.

Invention of various machines of war.
Archimedes anticipated by 1800 years many developments in the
calculus.