Which villain is the archenemy of the Green Lantern?

Sinestro is the archenemy of the Green Lantern.

Thaal Sinestro was introduced in Green Lantern #7 in 1961. He was originally a respected member of the Green Lantern Corps from the planet Korugar and became a mentor to Hal Jordan when Jordan joined the Corps. Sinestro’s belief in maintaining order through fear eventually brought him into conflict with the Guardians of the Universe.

After being expelled from the Corps, Sinestro was exiled to the antimatter universe, where he allied with the Weaponers of Qward. They created a yellow power ring for him, giving him abilities similar to those of a Green Lantern. He later founded the Sinestro Corps, whose yellow rings draw power from fear.

The main confusion is between Sinestro and other Green Lantern villains such as Parallax, Atrocitus, or Black Hand. Those characters are major enemies, but Sinestro is the enduring classic archenemy most directly associated with Green Lantern, especially Hal Jordan. His former friendship and mentorship with Jordan make their rivalry particularly personal.