Female wise, the 3rd female has the best form. All her fins meet and her anal doesn't extend past the bottom of her caudal. Her dorsal is wide, and her caudal had nice sharp edges.
The topline can have some curvature to it, but no bumps/dents.
Ray's must be even, say you have an 8 ray female, every main ray on her caudal must branch into 8 sections.
The best way to learn about form is to join the IBC. They have all their standards written in their yahoo group, but you must be a member to join it. There's a different standard for each tail type so it's hard to explain them all. You may be able to find them somewhere else too
Now, I know nothing of show standards, at all. But the first girl Feng posted has that little upward swoop by her tail. it's pretty common, and often (though not always) goes with a short bodied fish.
Just my opinion, I'd select away from that.
Out of those girls, I'd rank them 3,4,2 and 1. I have no idea about rays or anything, but I do look for even fins and a streamlined body. 3 seems the most even all around to me and the body most like the wild type
2 I think has better fins then 4, but she seems to get narrow in the body towards the tail, and not just because she's eggy. See how on 4, if you go from the front of the dorsal fin back, the lines of the body form smooth, V lines? And see how on 2, that same section has a curve?
I'd worry that if a male from her had that narrowness, he wouldn't be able to carry his fins or move as well and a male from 4, so I'd look for that when breeding.
But again, I am a total novice. Does anyone else think it's silly that the IBC keeps the standards a secret, that you have to be a member to see them? Don't they want just anyone who is interested enough in bettas to go to their website to be able to actually learn something? I think it would increase interest and therefore membership.