What's this?
I don't really like this, you just have sonic running through a bunch of scenes for no reason then the user gets to choose the fate of sonic? As if we cared. You haven't really animated anything as it is all mostly tweened sprites and there is a real lack of story line.
I'd say steer clear from sprites for a little while, my general opinion is I don't like any sprite movies but I can respect really good ones as quality work without enjoying them, so I think you have to work really hard to make a sprite movie good.
But enough of that, you clearly show the ability to know how to use flash, try and steer clear of generic themes like sonic, go for something more exciting and interesting to the user. The best test to see if something is good is sit down and watch it, and ask yourself if you think people would find it interesting to watch irrelevant of the amount of effort you put in.
Anyway stick with flash and you could improve greatly.