The Green Lantern has been one of DC Comics' most important superheroes for a very long time. The golden age saw Alan Scott as the Green Lantern and then when the silver age began, it was Hal Jordan's time with the emerald ring. With Kyle Rayner, John Stewart, Guy Gardner, and Jessica Cruz following, Earthbound Green Lanterns have also become important.
Green Lantern has several villains, both on Earth and in the far reaches of space, that he battles on a regular basis. While some are dangerous, others are downright laughable.
Hector Hammond
Easily, the least cool villain that Green Lantern fights is Hector Hammond. Hammond first appeared in Green Lantern #5 1961, created by John Broome and Gil Kane, and was one of Hal Jordan's earliest enemies. He gained enhanced intellect which allowed him to utilize powerful psychic abilities like telekinesis and telepathy.
However, overtime this villain has become less of a threat and his menace has grown stale.
Doctor Polaris
Doctor Polaris is DC Comics' Master of Magnetism, making him the company's version of Magneto. He has been a Green Lantern villain since 1963, John Broome and Gil Kaine creating him in Green Lantern #21.
A scientist in DC's mainstream universe, Doctor Polaris ended up a villain for the Suicide Squad as well, but he was nowhere near as cool as others who fought those antiheroes and is mostly a laughing stock who loses more than he wins.
Parallax
Parallax is an unusual Green Lantern villain who causes many problems for the Corps. He is the physical manifestation of fear and his mere presence causes impurity in the power rings.
Parallax was responsible for one of Hal Jordan's lowest points, where he turned completely evil and became one of the universe's most dangerous villains. While not technically cool, Parallax did give Green Lantern one of his most memorable moments in comics.
Nekron
Nekron is one of the most powerful villains that Green Lantern ever dealt with. He is the Black Lantern Corps' leader and he rose many dead heroes from the grave to start the Blackest Night storyline.
As an undead skeleton, he does fairly well on the cool scale.
Black Hand
Black Hand is an older villain who made his debut in Green Lantern #29 by John Broome and Gil Kane in 1964. He served as the herald to Nekron in the Black Lantern Corps and as part of the Indigo Tribe.
He has the power to drain the life force from those around him and can also drain the powers from the power ring used by Green Lantern. It was Black Hand who served as the Black Lantern Corps leader during Blackest Knight.
Larfleeze
Larfleeze is the leader of the Orange Lantern Corps and has also gone by the name Agent Orange. He is a newer Green Lantern villain, debuting in Green Lantern Vol. 4 #25 in 2007 by Geoff Johns and Ethan Van Sciver.
Living on Okaara, he mostly tolerated the Green Lantern Corps members when they came into his solar system, but eventually ended up in a war with them. Eventually, the two sides stopped fighting and Larfleeze has worked with the heroes on occasion since that time.
Atrocitus
Atrocitus is the Red Lantern Corps leader and founder. The red power rings are fuelled by rage, and he has an intense hatred for the Guardians, whose Manhunter androids slaughtered Atrocitus' family. He made his first appearance during Geoff Johns and Ethan Van Sciver's run in Green Lantern Vol. 4 #25 in 2007.
Atrocitus is cooler than Larfleeze in almost every way because Larfleeze is mostly about greed, but Atrocitus is all about rage and anger. He is almost unstoppable and will fight until the end in every battle. His passion makes for some excellent fights and sequences.
Star Sapphire
While Hal Jordan is Earth's greatest Green Lantern, his former love interest Carol Ferris became an incredible hero in her own right. Making her debut in the Alan Scott Green Lantern days of All-Flash #32 by Robert Kanigher and Lee Elias in 1948, Carol came into her own as a strong female supporting character with Jordan and then almost eclipsed him when she became Star Sapphire.
Her ring uses the power of love, but she was actually a villain when she originally gained her powers. She now travels the universe, fighting for love, and there aren't many things cooler than that.
Fatality
Fatality is an alien villain and one of John Stewart's most dangerous foes. She started off as a murderous supervillain, killing several Green Lantern Corps members under the belief that John Stewart destroyed her home planet.
She was dangerous and pushed Stewart to the limit, but the two developed an interesting relationship along the way. Fatality finally redeemed herself when she joined the Star Sapphire Corps and began to fight for love across the universe.
Sinestro
The most famous and coolest Green Lantern villain is easily Sinestro. He is not only Hal Jordan's most dangerous enemy, but he is also someone who respects and likes Jordan deep down. He is the leader of the Sinestro Corps, and using a yellow power ring, he has the one thing that weakens all Green Lantern heroes.
What makes Sinestro so cool is that he is both a dangerous and deadly villain, but also one of the strongest ring bearers in the universe and someone who will do the right thing in the right circumstance. On occasion, he can also be a powerful ally.