As of right now we got our first 3ds Title to surpass 7mil which is an accomplishment (but not so much for Mario Games)! We have yet to have a 10mil seller which isn't a bad thing!!! So this Is where you decide who will hit the mark of 10 mil 1st!!! Then who will follow up next???
As of right now MK7 has a bundle but so does SM3DL thats ahead with half a mil sales!!! We have the new NSMB2 that can definitely surpass 10mil but will it be the 1st one? Other games to keep in mind are AC new leaf, SSB4(which unlikely have 10mil but hey we never had a handheld SSB yet so who knows), and games not even released or mention!
Here is the list as of now (vgchartz numbers)
Super Mario 3D Land 7.27
Mario Kart 7 6.71
New Super Mario Bros. 2 3.72
AC New Leaf 1.27 (released a month ago not released in any region except japan)
Good luck with your guesses!