Although he first appeared in Omega Men and has served as an opponent and unlikely ally to various different DC heroes, Lobo is most closely associated with Superman. The indestructible Czarnian bounty hunter is one of the few beings in the DC multiverse that can match the Man of Steel in a direct fight and while this has made him a recurring enemy of Superman's, Lobo's unpredictability and unique code of honor have also resulted in the two begrudgingly working together on several occasions. The history between the pair is long and complicated but a one-shot issue went back decades to depict their first meeting.
DC First: Superman/Lobo #1 was a one-shot by Lobo co-creator Keith Giffen, Cliff Rathburn, Rob Leigh, Jack Morelli, and Brad Anderson released in 2002. It revealed here that Superman and Lobo's relationship dates back to the mid-20th century. In 1952, two alien members of the Hegemony race, are performing an experiment on a water supply in Metropolis. The issue then flashes back to a few years prior. The Hegemony became interested in taking control of Earth after discovering that pollution in its water would allow them to create a highly addictive drug they could sell throughout the galaxy. Believing Superman to be the ruler of Earth they sent an agent to meet with him while he was working as Clark Kent at the Daily Planet. The Hegemony representative, which was really a weaponized robot, offered to buy Earth from Clark and when he refused it attacked him. When Superman defeated the robot the Hegemony leaders hired Lobo to kill him.
Lobo initially refused the contract but changed his mind when he heard that Superman was the last living Kryptonian. Lobo himself is also the last of his race, but in his case it's because he killed all the other Czarnians so that he would be unique. When he arrives on Earth he goes on a rampage to attract Superman's attention, eventually attacking a rave where Jimmy Olsen was on a date. While out to dinner with Lois Lane, Clark hears the commotion and Superman arrives on the scene to confront Lobo. True to form, he offers to talk first but Lobo attacks him almost immediately. During the ensuing battle both are surprised to find that their physical strength is virtually equal.
As they fight, the Hierarchy begins harvesting water from Earth using large machines. When Superman and Lobo wind up close to one of the machines, they are teleported to the Hierarchy home world. When they arrive, they are attacked by a group of security robots. Lobo takes this as a sign of the Hierarchy interfering with his mission, which outrages him, as he fears it will lessen his fearsome reputation. He tells Superman he's no longer interested in killing him and heads out to defeat the Hierarchy himself instead, although he promises not to kill them. Superman agrees to send Lobo's flying motorcycle to him through the teleporter upon being teleported back to Earth. Lobo states that he looks forward to finishing their fight sometime in the future, after which Clark ironically says, "I doubt our paths will cross again." An editor's note at the end of the issue explains that Lobo relieved himself in the water, which altered its toxin content, making it useless to the Hierarchy, explaining why they never appeared in later stories.
This one-shot serves as a perfect introduction to and encapsulation of Superman and Lobo's complicated dynamic. Both characters possess nearly equal power which means Lobo can cause serious problems for Clark, let alone the other, less powerful heroes he comes across.
At the same time, Lobo's code and gray morality mean that he's not a clear-cut villain and can even be helpful on occasion, although that does not seem to be the case in their upcoming confrontation in DC's Black Label imprint.