I would choose the boldest pup, but I'd have to see a LOT more puppy tests and get my own hands on the pups to choose. Here's a few tests I did with a litter at 7 weeks old. There was a huge sable boy, three black and tan (very dark) girls, one light black and tan girl, one sable girl, and an average-sized sable boy. I was leaning on the side of the largest boy or the largest girl, simply because they were the alphas of the puppy pack. But I wanted to see how they reacted to things. First thing I did was casually walked away from the pen, then I ran straight at it, stopped about 5 feet away and just stared at the pups. Biggest boy and smaller girl run off into puppy house. Smallest boy stares me down along with biggest girl with a little tail movement, and the other girls simply take a few steps back and wag their tails (they recognize me from weeks of contact). I then went inside and made up with the ones that ran away. Biggest female didn't want to come near me.
Next test I stood about 50 feet from pup's pen, and fired various firearms at targets. All puppies but smallest male run at sound of shotgun. Smallest male is alert but calm.
Next, I waited until they were all settled down and thought I wasn't around anymore. I snuck around the side of their pen and suddenly and very loudly went WOOOWOWOWOOF (the alarm bark of dogs), all puppies but smallest male and middle female fun. Biggest boy yelps and cries of fear inside puppy house.
I then do various regular tests (puppy hold, surface tests, etc) and choose smallest boy because he was consistant in his responses (calm and alert) and is the most social with other animals and people. He also had the best prey drive. This smallest male is my pup, Carbon.
Since I have owned him, he has already growl-barked deeply when my fiance would approach looking like a threat. He also still loves people, although he has to evalute them quickly before he says hi. He also has displayed a deep, calm bite on his tug toy and is already learning when to bite, and when not to. Sometimes it's not the size or first appearance of a pup that determines what is best for your needs. (Carbon will be training for ASR/PPD)