I'm also interested in this. I know not all breeders wait until the pups are older to place them. To me, it was important to make my own judgements on temperament when meeting the pups, and to get the breeder's opinion. But it's impossible to know if we picked the "best" one for our family because we didn't take any of the others home! I always wonder about the pups we didn't pick....
Well, you DO always have the final say, because if you don't feel a puppy is a good match and the breeder insists that this is the only puppy you can have, you can always leave without a puppy... either to wait for another litter or to find a different breeder. I think very few breeders would want you to go home with a puppy you weren't comfortable with.
As far as the "best" puppy, and whether (after the fact) another might have been a better choice, the minute the puppies leave the breeder and start their lives in their new homes, they are being influenced by different surroundings, personalities, training styles and abilities. Kodi is the "best" dog for me now, but if he had grown up with someone else? Probably not. He and his brother Jib were two peas in a pod looks-wise, and the differences between them temperamentally were very subtle. Pam and I had actually talked about me taking Jib before I met them. Once I was there, though, there was just an immediate connection between Kodi and me and he ended up being the puppy who came home with me. That said, if Kodi hadn't been there, I am SURE that by now, with the time and training and just getting to know each other, I would be every bit as attached to Jib as I now am to Kodi.
As long as the puppy has the right energy level and basic disposition to fit into your family and training plans (if you have any) I think they grow INTO the "best dog for me" status. Otherwise, everyone on the forum would want Kodi!