I -always- go for activity, in that they respond, like Choclate said. It's the only time I appreciate getting flared at
Also, choose a color you like, and a fin-type you like. I'm not partial to those double-tailed ones - they creep me out, so obviously I wouldn't love it like my loves.
But on top of that, make sure they're not deformed! This is -really- important of pet-store bettas, and fish in general.
Size is not really important, but they should be well-proportioned. Some have big heads and short bodies, others have outrageous fins and bodies too small to really hold them up.
Curled fins won't ever straighten out, so choose whether or not to accept those, and oftentimes, they'll have mismatched "arm-fins" (I forgot the proper name like 12 seconds ago!).
Check for kinked spines and funny eyes (I've seen some weird things with fishes' eyes in pet shops) and anything else you can think of, then make a rational decision, 'cause there's nothing worse than having them die in a week, or never really perk up to having their own home.