Could be a lot better.
After watching this whole series and going through several pages of reviews, I saw far too many 10s, all of them undeserved.
Sprites - The actual edits weren't too terribly bad, but the recoloring was just atrocious. Like Metagen's original form. I think I saw at least 12 different colors on him. Ultimate/Final Form Sigma had a decent shape, but his colors were too heavy, for lack of a beter word. By that I mean that it was one shade of black all over, one shade of red, etc. Try experimenting with different shades to make it look more professional, and not like a simple paint bucket coloring. The fights definitely had a lot of action, but most of it was just brightly colored energy blasts and 5,000 sword slashes. Vary it up a little!
Sounds - WAY TOO LOUD. Also, you used the same sounds too often. There are much more than one impact sound, one energy blast sound, one screaming sound, do some searching! When I was looking for sounds for the flash I'm working on, I found at least 6 different punching and kicking sounds at ESF World Networkxx, and I plan to use each and every one of them. The music, while some of them were good choices (I almost used the Dimmu Borgir song myself) was drowned out by all the screaming and energy blasts. As for the voice acting, let's put it this way. I turned down my speakers during all the talking scenes. If I were you I'd just stick to text boxes, unless you can get some damn good actors. (WHY WAS DRAGOON GERMAN.) On a side note, please, PLEASE work on your writing. I had to get up and get some crackers for all the cheese. Also, profanity doesn't make a movie better. When's the last time you heard Knuckles call someone a bitch out of nowhere?
You definitely have potential as a sprite artist, but you've still got a ways to go before you're on par with the likes of Alvin-Earthworm and Proxicide.