Number 2, 5, and 6 are very clear and dynamic. I am able to easily read the flow of the poses.
Number 4's pose is a bit hard to read, one rule of thumb I tend to follow for the torso's line of action is to avoid "S" curves because they are harder to read.
If you want to capture the curve of the spine, my opinion is to add it after drawing a simpler line of action for the torso that is easier to read/draw.
Additionally, if you want more confident lines or elipses, do your best to avoid redrawing a line you already drew.
It might be lazy to just post this picture but i think this would also help illustruate what you might mean.
