It's difficult to definitively say who was the most famous scientist of the Hellenistic era, as fame is subjective and many brilliant minds flourished during that time. However, some of the most renowned and influential scientists from that period include:

* Archimedes: A towering figure in mathematics, physics, and engineering. He made groundbreaking contributions to geometry, mechanics, and hydrostatics. His inventions like the Archimedes screw and his work on buoyancy are still studied today.

* Euclid: Known as the "Father of Geometry," he wrote "Elements," one of the most influential mathematical texts ever written. His axiomatic approach to geometry laid the foundation for much of modern mathematics.

* Eratosthenes: A brilliant polymath who calculated the circumference of the Earth remarkably accurately. He also made significant contributions to astronomy, geography, and philosophy.

* Hipparchus: Considered the greatest astronomer of antiquity, he developed the first star catalog, accurately determined the length of the year, and discovered the precession of the equinoxes.

* Galen: A physician and philosopher whose anatomical and physiological studies dominated medical thought for centuries. His works were studied and cited until the Renaissance.

Ultimately, the choice of the "most famous" depends on your criteria for fame. All these individuals made profound contributions to science, and their work continues to influence us today.