Hum, this is a toughie.
They are all my family no doubt about it, and sometimes I wish they were the only ones I lived with, because some times, to be honest, humans just tick me off!
But, I do have a human best friend, and she's great, and if I was asked to choose my dogs or her, I couldn't do it. If I had to save one from a burning building, I'd run in the building and stay there... Cuase there is just no way I could choose my dogs over my best friend, or my best friend over my dogs..
So, yes, in a way, my dogs are my best friends, but they are my best dog friends, while I do have a best human friend, and I COULD NOT choose one over the other....