Superman III is one of my guilty little pleasures.
I also think that Reeve was in his best shape for that role too. He must have been around 230-235 pounds there.
The Superhero physique is important, to me as a fan anyway, and that was the closest I feel that he got to being in the right kind of shape I would expect for superman.
Some of the villains etc... were cheesy, but overall I think the movie is like the "cult classic" of Superman films.
Although they did not have Luthor as the villain, the other guy (cannot remember his name) was sort of a Corporate villain, and the corporate version of Lex always made more sense to me as a villain than the underground criminal.