Stand back and watch them for a little while. See which are the most active. Stay clear of that one, he may be sick or just plain shy.
Many people are fond of the runt of the litter.
Again, this may or may not be the best choice. Look for bright eyes and healthy coat. The fur doesn't have to be shiny, just thick and resilient. Ask to see the teeth or check them yourself. They should close like people teeth, top over bottom.