The Guardians of the Universe chose their Green Lantern Corps members based on several requirements: those worthy must have strong wills and must possess the ability to overcome great fear. While the majority of Green Lantern members are heroes like Hal Jordan, Kyle Rayner and John Stewart, the power rings have chosen several villains like Sinestro and Harley Quinn.
Additionally, many DC Comics villains deserve to wear power rings, showcasing their humanity, allowing their heroic traits to eclipse their villainous ones. Several Batman, Flash and Green Lantern villains haven’t yet donned Green Lantern power rings in the comics but deserve to nonetheless.
10 Sinestro
In his youth, Sinestro was worthy of a Green Lantern ring. The Guardians selected him, and he used the power of will to protect Korugar and his entire home sector, earning the title of "the greatest Green Lantern." Unfortunately, power and authority eclipsed his heroism, and Sinestro ruled over his planet.
Though Sinestro fell to fear's corruption, he is the villain who deserves a Green Lantern ring the most.
Sinestro experienced a brief redemptive arc in the New 52 Green Lantern comic when he received a new green power ring. Together, Sinestro and Hal Jordan defeated Black Hand and Volthoom. With the right opportunities and circumstances, Sinestro can actually be a powerful hero.
9 Harley Quinn
Surprisingly, Harley Quinn actually wielded a Green Lantern power ring during her Little Black Book miniseries where she teamed up with DC heroes like Power Girl and Hal Jordan. Harley uses Hal's power ring while he's unconscious to defeat invading aliens seeking an even more powerful ring. Harley was surprisingly composed and skillful with the ring, creating energy constructs like a giant green mallet.
While Harley acted as a villain in Gotham City alongside the Joker, she became an anti-hero with Poison Ivy and the Gotham City Sirens, even teaming up with Batman and the Bat Family during "The Joker War." She has the ability to overcome great fear and would make an excellent back-up Lantern.
8 Captain Cold
Captain Cold has never wielded a Lantern ring before, but he'd be worthy of a Green Lantern one. While Leonard Snart is a villain who leads the Rogues, he also commits crimes with a strict honor code that he enforces on other villains like Heatwave and Mirror Master. Cold will not kill innocent people and has even teamed up with the Flash on many occasions.
Cold also joined Lex Luthor's Justice League to repel the Crime Syndicate's invasions during Forever Evil. If Captain Cold possessed a Green Lantern ring, he'd likely turn over a new leaf and join Barry Allen and Hal Jordan.
7 Volthoom
Voltoom debuted during Geoff Johns' final arc on the Green Lantern book, forcing all the Lantern Corps, Hal Jordan and Sinestro to team up to defeat him. Voltoom is an incredibly powerful force who controlled the powers of all emotions on the spectrum, creating a universal threat that rivaled Nekron and the Black Lantern Corps.
The Guardians know Volthoom as "The First Lantern" because he was the first being in the multiverse to wear a power ring. Volthoom is "worthy" by default thanks to his ability to create his own power rings. Volthoom could easily craft his own Green Lantern ring considering he can wield the power of will already.
6 Star Sapphire (Carol Ferris)
Carol Ferris was a longtime Green Lantern villain, corrupted by the Star Sapphires' chaotic, controlling view of love. Her tenure as the villainous Star Sapphire strained her on-again/off-again relationship with Hal Jordan, but once she controlled the Sapphire, and gained a new violet power ring of love, Carol became one of Hal's greatest allies during events like Blackest Night and Brightest Day.
Carol is an experienced ring-bearer thanks to her years as Star Sapphire and could easily substitute violet for green to become a Green Lantern. If Hal ever lost his own power ring, Carol could definitely take it and fill his shoes.
5 Deadshot
Deadshot has also never wielded a power ring, but he could be worthy of one. Deadshot lives and works by codes. He's also helped save the world several times as a member of the Suicide Squad. Heroes can convince him to help, as long as the outcome benefits Deadshot in some way.
A Green Lantern ring could act as a catalyst for Deadshot's full redemption arc. With its power, Deadshot could right all of his past wrongs, using the power ring to save lives rather than take them. Additionally, Deadshot would make a powerful Green Lantern. John Stewart fuels his energy constructs with his training as a soldier and Marine. A Green Lantern Deadshot, with his precision, would be incredibly effective at long-range.
4 Hal Jordan (Parallax)
Hal Jordan usurped Sinestro's title as "the greatest Green Lantern." Hal Jordan helped launch DC's Silver Age, founded the Justice League and saved the world many times during events like Zero Hour, Blackest Night and "War of the Green Lanterns." However, forces like Parallax have corrupted Hal in the past. Through Hal, Parallax, otherwise known as the Entity of Fear, could wield the power of a Green Lantern ring.
Under Parallax's control, Hal actually obtained many GL rings, combining their energy to gain a massive power boost. Parallax already has wielded willpower many times after corrupting Hal, Kyle Rayner, and even the Guardian Ganthet. Parallax is a parasite and could cheat the system and become "worthy" of a GL ring by infecting and manipulating Corps members.
3 Darkseid
In reality, the Green Lantern Corps would never allow Darkseid to join their ranks. Darkseid rules Apokolips and constantly invades New Genesis and Earth. On paper, however, Darkseid is fearless and strong-willed. If Darkseid can control the Anti-Life Equation, he can certainly power a Green Lantern ring.
A villain like Darkseid joining a heroic team is not unheard of. In Marvel Comics, Apocalypse temporarily joined the X-Men and mutants on Krakoa, even saving their lives during X of Swords. If Darkseid gained a power ring, he may actually help the Green Lantern Corps... until he inevitably betrays them, of course.
2 Power Ring
Power Ring debuted in the Silver Age as an evil counterpart to Hal Jordan's Green Lantern. On the backwards Earth-3, the Crime Syndicate were villainous versions of the Justice League. The Syndicate returned in the New 52 Forever Evil event where Geoff Johns retconned some of their traits and backstories.
This new version of Power Ring wore a ring crafted by Volthoom, the First Lantern rather than a corrupted Green lantern ring from the Guardians. Power Ring was an incredibly cowardly character and wouldn't usually join the GL Corps, but with assistance from someone like Volthoom, Power Ring could fake worthiness of a Green Lantern ring.
1 Dawnbreaker Batman
The Batman of Earth -32 debuted during the Dark Nights: Metal event as a Batman of another universe who earned a Green Lantern ring after his parents died. This Bruce Wayne overrode the Green Lantern ring failsafe, using it to kill Joe Chill and many subsequent Gotham City villains. Eventually, Bruce became Batman: The Dawnbreaker, combining the darkest elements of Lanterns and Batman.
This Bruce Wayne was a villain who maintained control over the power ring. While he didn't meet any of the standard heroic requirements of Green Lantern Corps membership, Batman: The Dawnbreaker certainly had tremendous willpower and could overcome great fear, earning the Green Lantern power ring nonetheless.