Obviously the Cornish X, like everyone else said. But if you are asking about a dual purpose/heritage type, I had great luck with Australorps on pasture. Partial pasture actually, I only let them out of the run in the afternoons and they cooped back up at night. They get to be a good size, grow very quickly, and I think they hit the happy medium between foraging ability and activity level, so they don't just burn off the calories they get on pasture.
I'm sure there are other breeds that do as well, but these are just the ones I have the most experience with.