Define "must-have". If you mean just pure quality/enjoyment, then it'll vary from person to person. Me, currently, it's Kirby: Triple Deluxe, but that can change VERY easily looking at the 3DS backlog that I have. As far as SHOWCASING what the 3DS is all about, though? In my opinion, no game does that job better than Kid Icarus: Uprising. Say what you will about it, heck, it's not in my top 5 3DS titles, but nearly everything that it has is representative well as far as showcasing various aspects of the 3DS, from streetpass to AR cards to the system control usage (weird name, but I'll explain), and of course, 3D. While other games may do individual aspects better (in my opinion, I think Bravely Default makes better usage of Streetpass better than Kid Icarus does), no other game's...average score ranks as high as Kid Icarus does as far as representing the 3DS.
Streetpass is actually one of the "weaker" aspects, merely allowing you to swap weapon gems, but for one, it's way better than, say, Pokemon X and Y's "streetpass functionality", and for another, it is a showcase of one of the major systems in Kid Icarus: Uprising, which is in relation to weapons and the customizing that it offers. Also, it uses play coins, which from my admittedly limited time with 3DS titles, not many games use play coins.
AR Cards. Ho boy, now THIS is where KI shines. There are over 400 AR cards to get (good luck getting them "normally" in America, though) and they can be used to either unlock models or interact with one another in various AR card battles. The extensiveness of this system helps KI rise greatly in the showcasing criteria that I've placed.
System Control Usage. While the ground controls are a pain in the butt, not only are they one of the "best" solutions the developers could come up with in regards to the system itself, assuming that the actual mechanics don't change, but they make great usage of all the buttons and sticks on the 3DS. Now that may sound like a bad thing to some people, but for others, it helps to showcase the input placement for the 3DS.
3D. The 3D in Kid Icarus is one of the best. Not much else to add here.
But yeah, bottom line, if I had to pick one game to showcase it across malls and such, it'd be Kid Icarus: Uprising. Of course, the game itself may not appeal to everybody, so I'll have other games that are either personal favorites or other showcase games that also represent the 3DS really REALLY well. ...or maybe games that are both. --- 7 is good. I swear the school's 75-100 grading scale has ruined peoples perception of things. -MillionGunmannn