I'm going to put my vote in for Porcelain D'Uccles. We've had a few and every one has been actively friendly, enjoying and seeking out attention. Sadly, this can make them very prone to predators and accidents, so I think if you want one as a pet, make it a housebird with a nice little indoor coop (rabbit or parrot cage).
Bantam games come in a close second, although they seem to need more attention as (ridiculously tiny!!!) chicks to be as friendly as D'Uccles.
I haven't read through, but I'm sure a LOT of people have voted for Silkies. In my experience, they are very tolerant of people and handling, but that's not the same as actually friendly. We just lost our Silkie hen who was a house bird for the past 2 winters, as she was not a hardy bird, and even though we enjoyed her toodling around the house, and she'd always let herself be picked up and handled, she never sought us out.
For big breeds, I have to say Buff Orps have been the most consistently friendly without special treatment, Some of them just, right from the brooder with other chicks, decide the Big Thing That Brings Food is awesome and they're going to make friends. But, having said that, the friendliest chicken we ever had that wasn't meant to be a special pet and so got special treatment was an EE hen, that adopted DD, would hop in her bedroom window, have a cuddle on the bed, and lay an egg in her hamper. I really miss that chicken. But we've had a LOT of EEs who've run the gamut in personality, so I can't say as a breed they are consistently friendly, but as a breed they do seem to be consistently bold and curious, so it wouldn't surprise me if they also had a bunch of votes.
.Whenever I returned, they would utter some innocent chirps and stare at me with virtuous eyes. The buff orpingtons were good, however.
These little ones are 6 weeks old and have access to a huge run, but chose to come sit with me instead. 
All the other chickens ran away from me and are happily foraging.
