When you say the best at free ranging, do you mean the best at foraging, the best at staying alive or both?
I think most breeds are very good at foraging. I've always just gone with what I've liked, that does well in our cold winters and has a good temperament. They've all been great foragers. Chickens love to eat! The more they forage, the better they get at it. It takes them a little time to find out what's good to eat, if they don't have another chicken to show them. Every plant, bug and worm they eat, is a new experiment for them.
The Cornish crosses bred as meat chickens are probably the only really bad foragers. They have a hard time just getting around as they grow. Breeds that have trouble seeing where they're going don't always get around as well. You can always give them a bit of a trim around the eyes, though, so they can see better.
Are predators going to be a big problem where you live? I think Polish are going to have a harder time, unless you trim the feathers that cover their eyes. They just won't be able to see what's coming. How well they do, might just depend on your particular situation. I've always heard and read that white chickens get taken by preditors more often, so have stayed away from them. I don't know if that's actually true or a myth. I think the lighter, flightier breeds are supposed to stay alive better when it comes to predators.
A lot of people free range the heavy breeds, though, including me. Orpingtons and australorps are wonderful chickens. So are any of the varieties of rocks. Sussex, Dorkings and Faverolles are also very nice chickens.