So far, from my flock, I'd say SILKIES!!! I have both Cochins and a single Silkie. All are about the same age; the Cochins are about two months older. However, only the Silkie has become broody. In fact, I have some eggs under my Silkie due to hatch this weekend.
I am concerned about the Cochins that I have though since they are some mean little buggers! Even as chicks, they would attack younger chicks. I may have to isolate them when I'm ready to let Momma and babies join the rest of the flock.
I have had a Barred Rock go broody in the past as well. Sweet breed and by far, has been my friendliest.