Quite clearly Arthur; Garfield might have outpaced him had he lived longer (curse you, 19th century doctors).
My top three:
1. Cleveland
2. McKinley
3. Arthur
Um, you know Cleveland's history with organized labor, right? Not that the Gilded Age presidents are known for their pro-union sympathies, but Cleveland's second term was particularly dark on that count.
I'm willing to overlook his intervention in the Pullman strike and overall economic policies because I'm a big fan of his foreign policy especially regarding Hawaii and the Venezuela Crisis. I guess the Pullman strike would be a deal breaker for most socialists, but I'm not a huge ideologue.