Scarecrow, as Yellow Lantern, used the fear energy of his Sinestro Corps ring to terrify innocent people into committing violent crimes, an upgrade on his infamous ability to terrorize Gotham with his fear gas. However, without these things, Scarecrow is no longer terrifying.
Test pilot Hal Jordan went from being a novelty, the first-ever human Green Lantern, to one of the most legendary Lanterns to ever wield a power ring. Hal Jordan's life was changed twice by crashing aircraft.
The yellow weakness was cause by the creature Parallex that the guardians had imprisoned within the central power battery. It a yellow entity, yellow being the color of fear in the emotional spectrum. Once he escaped there was no more yellow weakness.
It is more than a desire for revenge against the Green Lantern Corps that has caused Sinestro to especially hate and strike at Hal Jordan. It is far more personal. Jordan was not corrupted by the ring as Sinestro was. Jordan was hailed as the greatest Green Lantern of them all.
1 Sadam Yat. Sadam Yat was prophesized to be the most powerful Green Lantern to ever live; however, Sadam was also said to bring forth the end of the Green Lantern Corps.
Hal Jordan is regarded by many as the greatest Green Lantern of all time. While the Corps is filled with some of the most worthy beings across the vast DC Universe, Hal stands above his peers as one of the best ring-slingers around.
Sinestro managed to convince Hal Jordan that fear is more effective than will power, thus making him give up the power of the Green Lantern and accept the yellow ring of fear, joining the Sinestro Corps.
On top of that, his wife was killed by the Manhunters, who were the predecessors to the Green Lanterns and were created by the guardians. This drives him mad and he becomes a tyrannical dictator trying to re-establish order to save his people by any means necessary.
Unfortunately, Hal Jordan turned out to be more than a match for Sinestro. The two of them fought with their power rings until it came down to a brawl with bare fists on Oa. The fight came down to Hal getting Sinestro in a headlock and snapping his neck.
It's the effect his costume and his reputation have on the criminals of Gotham City. However, because of his powerful will and because he had also been previously exposed to a Green Lantern ring, the Yellow power ring quickly left and chose to seek out another subject instead.
Considering that the Yellow Lanterns draw their powers from the universe's collective fear, it stands to reason that the Sinestro Corps is more powerful than the Green Lanterns.
Originally it was a 'yellow impurity' that was used to transform the blue mental energy of the Guardians into a physical form allowing them to create the Central Battery, the smaller batteries and the power rings. The impurity made the rings and their constructs unable to affect things that were yellow.
Even after Hal returned to Earth, the Guardians kept asking Jordan to put the Corps over his private life, abandoning his friends when they needed him most. Fed up with their apathy, Hal Jordan decided to retire from the Corps, and John Stewart became the official replacement for the Green Lantern of that sector.
He's powerful but not invincible, and there are ways to stop him. Hal Jordan, on the other hand, did not need any of these means to beat Superman. Instead, he uses the pure strength of the Green Lantern Corps, proving that a Green Lantern alone is powerful enough to defeat Superman.
While Hal Jordan and his unique Green Lantern power ring may be special, he no longer holds a candle to John Stewart who no longer needs a ring at all, being cosmically powered straight from the source.
As a Green Lantern, Hal Jordan is semi-invulnerable, capable of projecting hard-light constructions, flight, and utilizing various other abilities through his power ring which are only limited by his imagination and willpower. Jordan, as a Green Lantern, has exceptional willpower.
Space Ghost's intangibility allows him to avoid the best offense Green Lantern can muster. Even when Hal goes on the defense and forms a shield, Space Ghost effortlessly knocks him out with a concussive beam. While Space Ghost is enough to make Hal sweat, he'd probably be in danger with the Avengers' Vision as well.
What Makes Him Pure Evil? He instigated the years long Rann-Thanagar War through a series of false-flag attacks including destroying Rann's capital city and sabotaging any peace talks. One attack alone claimed hundreds of lives.
Sinestro didn't really turn evil. He became a villain. He wishes to impose order upon the universe. He might seem evil - he's manipulative and controlling - but at his heart, he has the best interests of everyone under his protection in his heart (most of all his own people and Korugar).
A recent Superman story shows that the Man of Steel was able to defeat both Sinestro and Parallax quite quickly (and without a Green Lantern's ring). Green Lantern has some of the most powerful and terrifying enemies in the DC Universe.
