Nintendo's studios are all pretty small, so I'd say none of them. Retro can make a great first-person game with shooting mechanics, they've proven that. But they haven't even touched online capabilities, and suggesting that they could tackle something on the scale of COD or Titanfall is laughable.
I voted for the Mario Kart team, because they are Nintendo's most experienced studio where online play is concerned. So maybe if Nintendo mashed Retro and the Mario Kart team together, that studio could do it... if they could overcome the fact that the two halves of the team couldn't communicate very easily since one half is American and the other half is Japanese.
With regards to the poll, there isn't really a "Smash Bros / Kid Icarus" team. There is only Sakurai, and he can do pretty much whatever he sets his mind to. But he isn't a Nintendo studio. He isn't even technically a Nintendo employee. He's freelance.
