Sonic, without a doubt...as long as he's playable from the get-go. I was hoping ever since Snake was announced that Sonic would be in it.
Runner-up: Metaknight
Im gonna use Samus or Ike first because both their final smashes are amazing. I also want to try out all the new characters especially metaknight, pit and snake. I also hope to use Captain Falcon if they put him in the game.