Lots of actors have played Superman, but Christopher Reeve embodied
Superman. Looks aside, no one else has ever captured how Superman should behave and carry himself quite so perfectly. He may not have been a perfect fit for the definitive Curt Swan Superman look of that era, but he superseded it and became an influence on how Superman has been depicted ever since (starting with Byrne).
Second place would be George Reeves, who was a very good fit to the stocky, square-jawed Superman of earlier decades.
Welling occasionally looks a bit like Superman -- his face reminds me of Jeff East from Superman: The Movie
, which I suspect may have been a factor in his casting -- but in his general bearing and performance and mumbly delivery, he's usually very far from being right for the part. He's been getting better lately, but not by much.
As I've said before, Routh would get my vote for looking (and sounding) the least
like Superman. Second-least would be Cain, who's just too soft-featured and high-voiced to resemble Superman that much. Cain also gets my vote for "worst job differentiating his two identities," aside from Welling, who's obviously out of the running in that category. Best job differentiating Clark and Superman is Reeve by a mile.