Mario Kart 7, with not only recent series developer Nintendo EAD but also Austin-based Retro Studios.
"But Retro Studios only worked on the Donkey Kong Country Returns level," you whimper. While that was true as of last E3, Hideki Konno, Manager and Producer at EAD Software Development Group No.1, tells Iwata that since several production designers were working to get Nintendogs + Cats ready for the 3DS launch, as well as other Nintendo projects, there was "an act of emergency" to get Mario Kart 7 finished by the end of the year.
Retro was brought in to help on some courses, notably the aforementioned DKC-themed level, and to work on the game's 16 classic courses, while EAD ostensibly worked on the 16 new tracks. With the 3DS still recovering from a rough launch, plagued by pricing concerns and a paucity of premium software, we'd say it was a smart move to ensure Mario Kart 7 would find its way onto shelves this year.