a couple of things to judge in your picking of a rooster. Check his feet, the toes should be straight, the legs should be strong and well developed. Watch the bird walk, again it should have an even gait. Pick the rooster up, and feel his body, find his breast bone, pull out each wing, again looking for conformity, balance and even both sides development. Check the beak, how it closes, it needs to align well. Look at the scull, is it wide, is it eve. The feathers should be bright and full.
One can check the standards of the breed, I learned a lot, by just reading an article on here about the standards of the Buck-eye breed. Each breed has a bit different requirements as to shape and color, but the above are good traits to look for in all animals if one is using them for breeding, one should start with a physically sound bird.
Or keep the one that is the nicest!
ps - I agree with pulling them out of the flock about 8-10 weeks of age. If you had a mature rooster over the flock, you could keep them a bit longer, but if this is a flock of chicks growing up together without any adult chickens to keep them in line, very often roosters get to be jerks.