I don't really know, there are lots of good characters... So, maybe Mii and Robin, because we finally have a character who have a moveset that mixes magic & physical damage, also, forgot to mention his Down + B attack that deals damage to enemy, and restores your health !
As for the Mii, because of the E3 trailer where Reggie fights Iwata with his Mii. They look balanced, if they're the good size.
And they have three movesets finally (though they can't change it in battle).
(PS: I didn't add Pit and Meta Knight because they are my SSBB favourites, so, I don't need to add them in, I know that I'll play as them anyway)