...okay, we all know what it is: Just Dance 2. Ubisoft announced that it was not only the best-selling third-party Wii game of June, according to NPD records, but also the best-selling third party Wii game in the US since its October debut (not that there's been much competition since October 2010). And it's the best-selling third-party Wii game in the US, ever.
The franchise (including both Just Dance games and Just Dance Kids) has shipped over 14 million copies worldwide. Ubisoft is celebrating the success by bundling all the Just Dance 2 DLC as a standalone game, Just Dance 2 Summer Party, coming out on July 19.