Whatever you may think of Reeve's build, his body language
captured Superman in a way no one else has approached. His ability to transform from Clark to Superman just by changing his posture and attitude was striking. And his experience as a glider pilot let him make the flying sequences convincing by giving the impression that he was actually shifting his weight and moving his arms to maneuver. Compare him to Terence Stamp, Sarah Douglas, and Jack O'Halloran in Superman II
. They just look like they're hanging from wires and sticking their arms out. Reeve looks like he's flying under his own power. That's why the slogan for the first movie was "You Will Believe a Man Can Fly."
Of course, few of the other Superman actors have really gotten as much of a chance to demonstrate flying skills, since the use of flight effects tended to be more limited in the various TV shows, whereas both Alyn and Routh had their flying shots done via animation. I guess there was a fair amount of flying in Lois & Clark
, particularly in later seasons, but it was generally the same principle as George Reeves used, lie the actor down on a plank in front of a screen -- except it was a greenscreen rather than a rear-projection screen. I'm sure they and Superboy
used some wire work, but I don't recall either show enough to say how much. But if the actors sold it at all, it was probably by emulating Reeve's precedent.