Worst. Flash. Ever.
I hate it. First of all, the use of sprites. LEARN TO DRAW! SPRITES ARE LAZY! Second of all, the 'metal' bowser was nothing more than the Photoshop 'Emboss' filter applied to Bowser's spritesheet. The effect looks terrible.
Okay, now onto the worst part of the movie: The plot. WHY MUST PEOPLE HAVE MARIO AND SONIC IN THE SAME FLASH?! What irks me more is the giant super-long fight scene that seems to go nowhere fast, and has no plot whatsoever. I've seen the 1st and 2nd ones, but this series takes sprite animation with new lows by ripping off one of the worst action series ever made. Yes, I'm talking about Dragon Ball Z, which had poorly-animated fights, a bad plot, and the Namek saga which had a 2-hour-long fight above a planet destined to explode in five minutes. It also rapes my favorite series ever, the original Dragon Ball, pre-Z.
Now, if I had to make it better, first of all I'd put in actual character animation. Sprites arent going to do it, unless you drew them yourselves or come from a videogame that hasn't been done a million times over.