Short of the obvious like Hearns, Robinson, Leonard, etc. I don't think there's a reason he couldn't beat anyone. Had the fight with Mickey Walker happened when it was scheduled, I'd have chosen Leonard.
The Mickey Walker fight would have been a mega fight. Not just in retrospect as 2 x fighters still typically ranked top 15 p4p all time even today, but in the context of the day, they were both big stars. I don't know who would have won, but Mickey was naturally a good deal bigger (a couple of inches taller and much thicker set).
I would certainly take Benny Leonard over guys like Porter, Thurman, and Spence. I think he has an excellent chance of beating the Welterweight version of Mayweather. Not sure I would favor him over a guy like Trinidad at Welter.