Easy. Mickey Walker. Vastly more impressive against significantly larger world class opposition and nothing like as inconsistent. Britton's win resume is immense, but he lost a lot, plenty to non elite opposition.
Both top 10 all time welterweights but Walker had success above that weight and was better in a p4p sense.
Walker decisively beat Britton H2H (second time) although Jack was past his best.. Both ATGs but in a P4P sense the obvious answer is Walker who became a legitimate HW contender despite being 5'7 & rarely weighing over 170lbs .. Walker is in my top 10 fighters of all time, his career was very unique.