While Marc Spector's Moon Knight may be a part of the Marvel Universe, his ultimate form would be as a Yellow Lantern from DC Comics. A former agent of the Egyptian moon god Khonshu, Moon Knight's brutality is relentless as he protects his territory in New York, hardly showing any restraint as he beats and intimidates any who dare oppose him. Using fear as his primary tool, it's not out of the realm of possibility to suggest that a yellow power ring would find him if he were ever to step foot in the DC Universe.
Harnessing the yellow light of fear, the Sinestro Corps rival the Green Lanterns and their use of green willpower, and has just as many officers in the service of Sinestro himself, the first to wield fear after being rejected by the Green Lanterns. Chosen for their ability to inspire fear, those recruited into the Sinestro Corps are some of the most formidable beings in the universe. In recent comics, they've risen to greater prominence thanks to the Green Lanterns being almost destroyed.
It certainly seems as though Marc Spector would fit the bill to become a Yellow Lantern. In past Moon Knight comics, Spector explains that the entire reason his costume is white is so that the criminals he hunts can see him coming from a long way off, wanting them to be good and scared by the time he's close enough to serve up his brutal brand of justice. As such, becoming a Yellow Lantern due to his great capacity to inspire fear makes a lot of sense, resulting in a Moon Knight that would be incredibly powerful.
Interestingly enough, it's recently been confirmed that a Yellow Lantern doesn't necessarily have to be a villain. With the Green Lantern Corps near destruction in recent comics, the ring of former Green Lantern Jessica Cruz was depleted of its power. In response, she's chosen to try her hand at wielding fear as a Yellow Lantern to continue doing good. As such, it seems as though the light of fear isn't inherently corruptive to one's goals, meaning that Marc Spector could still dispense justice as a Yellow Lantern.
Moon Knight has long been compared to Batman, who himself was once selected to wield a yellow power ring when the Sinestro Corps was first formed (despite his rejection). It stands to reason that the same recruitment opportunity to become a Yellow Lantern would be presented to Spector if he ever crossed over to the DC Universe. It would certainly be the most fearsome and powerful form Moon Knight would ever take.