I would say the RC-3 is the most versatile for every day carry. It is small, making it more likely to be carried around. Has the thinnest stock and grind making it the best cutter of the bunch.
The RC-4 is well suited to bushcraft with extra girth and thickness for wood working but still handy enough for game. It is pretty much a perfect survival knife, the kind of thing that is bullet proof in construction but small and light enough to keep with you at all times.
The RC-6 is more specialized as a camp knife, better for splitting larger pieces of wood and heavier tasks. The RC-6 definately has the cool, big knife part going for it but you aren't likely to be strapping this beast to your hip in the city.
I currently have the RC-4 and love it as an all around camp knife. I have an RC-6 on pre-order and am aching to get my hands on an RC-3...
Then there is the Izula - the perfect neck knife....