Out of all the Flash series on newgrounds based around Mario and/or Sonic, the best one by far is the Mario Bros Z series by Alvin Earthworm... Recently Alvin hasn't been up to continuing right at the moment, but was said to continue once he gets in the mood again...

This movie is 30 minutes long, and takes about a minute to get right into the action... This is also Episode six of the series so if you don't know what is going on, I would strongly advise to watch parts 1-5 to understand what is happening... Other than that, sit back, relax, and enjoy the show...