Which ancient Greek astronomer is the earliest known proposer of a heliocentric model?

The earliest known proposer of a heliocentric model was Aristarchus of Samos.

Aristarchus, a Greek astronomer and mathematician active in the third century BC, proposed that the Sun remained still while Earth revolved around it annually and rotated on its axis daily. His original work on this model has not survived, but Archimedes described it in The Sand-Reckoner.

Aristarchus also argued that the fixed stars were extremely distant. That idea offered an explanation for why ancient observers could not detect stellar parallax as Earth moved through space. He estimated the relative sizes and distances of the Sun and Moon using geometric observations, although his numerical estimates were far less accurate than modern measurements.

The model did not become the dominant ancient view. Aristotle and Ptolemy supported geocentric systems, and heliocentrism was revived much later by Nicolaus Copernicus, then refined by Johannes Kepler and explained physically by Isaac Newton. Aristarchus is therefore the earliest known proposer, not the inventor of the modern complete theory of planetary motion.