The most characteristic product of Greek mathematics may be the theory of conic sections, which was largely developed in the Hellenistic period, starting with the work of Menaechmus and perfected primarily under Apollonius in his work Conics.
The three classical Greek problems were problems of geometry: doubling the cube, angle trisection, and squaring a circle.
Pythagoras: Famous for the Pythagorean theorem and his contributions to number theory and geometry.
Euclid: Often referred to as the "father of geometry" for his work in the Elements, a comprehensive compilation of geometric knowledge.