Metal Mario, hands down. There's a reason he hadn't appeared since SSBM before this game, people! (Well, as a character, not power-up.) And not only that, but his silence in SSBM made him like a cool, mysterious character. The fact that he sounds like Mario with that sound effect over top is stupid. If he acted like a robot, he'd be 100x cooler and probably not my least favourite newcomer.
Next on my list would be Lakitu and Queen Bee. I like Queen Bee, but I don't think she was necessary to be added in yet. Maybe they should've waited for something like Mario Kart Wii U that will likely have a large roster. Lakitu, on the other hand, has a terribly annoying voice, but I can see why he would be included.
And then there's Wiggler, one of my favourite characters in the whole series, and I especially love how he gets angry when he's hit with an item.