Pineapple on 01 January 2013
The 3DS, with Mario Kart 7, seems the most likely to me. The other option is Nintendo Land. There's no other released game that I view as anywhere near as likely to make it.
Someone mentioned Diamond/Pearl, but that's just not going to happen. It needs slightly over 100,000, but in 2012, it sold just 85,000. The chances of it selling close to that in 2013 are very slim. The only way it's going to make 18.15 million is by getting adjusted upwards, which seems very unlikely at this time.
Another option would be Blizzard releasing data for total physical copies sold of World of Warcraft, and that number exceeding 18.15 million. The game obviously beats that in total, but quite a bit of that is digital, which VGChartz wouldn't add.
