Poses and wardrobe in #1-3 are SUPER-awkward. The open-shirt is creepy.
From a photographic standpoint - dark hair on dark background tends to disappear. Notice how in #2, it stands out better from the background. So you can either pose your subject against a lighter background, or use some rim-lighting.
#4 is not bad, though she has a weird shadow across her face. It doesn't ruin the picture, though
#5 looks like a snapshot and I'm not sure if the eyes are sharp, since they're hard to see at that size.
#6 probably has the best pose, and best feel...though the arm extending directly out of the side of the frame is weird, since it has no context. (I assume he has his hand on a tree or something.) Eyes are nice and sharp, always a key for a portrait.