Swan is, for my money, the best Superman artist, with the single possible exception of Neal Adams.
Swan is clearly the definitive, but I'm not sure I'd call him the best. After the early 70s he stopped trying to update his work, he was saddled with a lot of crappy inkers and his stuff really suffered as a result.
For my money, one the best Superman artists pre-Crisis was Jose Luis Garcia Lopez. However, for some reason, they only used him on a handful of fill-ins and covers and then sent him off to DC's editorial staff to design all the model sheets, etc.
I always thought it was a shame they never gave him an extended run on Superman.
