The shading is really basic. And I really mean it, because there isn't much to it other than just filling colors inside the outline of the sprites. The sprite style could certainly use a lot of more anti aliasing around the outlines, to avoid looking so generic. The eyes also look pretty emotionless, given that his pupils are just black squares without any detail to them. Also, forgot to mention that some poses in here seem to have a really thin and small body, given that there are times that you can only mostly see his head and not much else. Sonic also looks pretty calm when he dies and drowns.

As for the palette, the skin shading has a really slow contrast, given that the mid tone is really difficult to tell apart, and the shading on the white also feels kind of weak. For Super Sonic, I suggest that you go for a different tone of yellow that is more gold looking.

I'll fix the animations that don't look right( like walking and ducking ), maybe add some new ones, add a few more head angles( like waiting; he'll actually face the camera now ) and probably make sheets for the other characters.
Also, what do you think of the shading and color palletes?

The animations... look really stiff in most the sheet. The fact that his head angle stays the same through most of the animations is a big letdown, and doesn't help making him seem that much of a dynamic character either. In fact, it bothers me even more that his head is facing the camera more than actually facing forward.