Epic in the Extreme!
Brutal fight scenes, dynamic action sequences, highly detailed special effects, a rapid but traceable story pacing; if it weren't done with Sprites, I'd call this professional! The musical selection was impeccable, especially with your usage of Snappleman's rendition of The Doomsday from Project Chaos--which in itself is an extremely BADASS element.
One thing that particularly satisfies me about what you've done here, though, is the dialoge. You've captured the characters perfectly; from the pantomime sibling hijinks of the Mario Brothers to the corny alliterative/rhyming banter of classic super mario villains, from Sonic's wildcard, loose-canon quips to Shadow's cold, begrudged aloofness, even the evil computerized cackles of Mecha could not have been placed or written better than a professional staff at a licensed production firm.
I feel that you've produced here perhaps the most complete, detailed, and in my opinion flattering crossover story arc of Sonic/Mario ever to be seen. Their powers are outlined with fine attention. Your emphasis on Sonic's speed was absolutely spot-on. The coverage you gave to the Blue Blur provided much more immensity to the battleground, making the island feel truly like an entire self-sustained ecosystem covering all extents of habitat. Mario's stunts were particularly remarkable, when we got to witness the effect of the fireflower, the donkey kong golden hammer, and the various martial arts capabilities he's shown us in his games. Yes, you made these characters powerful and articulate--but you didn't go even a STEP out of bounds of what one could come to expect.
Last of all, your sense of humor with this piece was tremendously well-timed, giving us a breather of a good laugh at just the right times to take a peek at what's going on with Wario and Waluigi, and various other slap shtick elements that allowed us to keep the moments of outrageous ASS-KICK in perspective. Really, taking a moment every now and then to show just how the battle was humorously affecting the bystander party (wario etc.) was the ideal balance to the actual combat.
All this, and there's still episode SEVEN coming up! Simply astounding!
In my personal opinion,
Super Mario Bros. Z is one of the few things I still bother to look at newgrounds for these days.
Happy Flashing!