Is free-range chicken more flavourful?
Flavor in most meats is generally determined by a few things.  


How the animal was raised and if it is of a particular heritage breed.
How it was treated over its lifetime (i.e. diet and environment).
When and how it is slaughtered.
How it is handled from the slaughterhouse to the butcher, was it properly stored, etc.  


Also, how fatty the bird is and light to dark meat ratio will also determine flavor of the bird.  

I definitely have tried a lot of different types of "free range", "organic", and/or "local" chickens out there and you basically get what you pay for in terms of meat quality and flavor. At my restaurant, we try to source best quality chickens — local, organic and free range, if possible — because it just tastes better and it coincides with our philosophy.  Properly cooking and seasoning the chicken will also help.