Every 3DS needs Smash. And Animal Crossing: New Leaf. But that's not for now.
Stages like Balloon Fight and Magicant are amazing, Arena Ferox and Yoshi's Island (Brawl) are great in competitive, all the DLC is accounted for and even if you got Smash U, 3DS is great for practice.
Smash Run needs absolutely no introduction. I'm sure dozens of others can do a much better job at explaining its greatness in full. I'm not good with words. But when people say it never gets old, it really doesn't. You got 56 Characters so far, countless possibilities of Powers and Custom Moves, the countless Events, Layouts and Final Battles... it's a bit baffling.
3DS Classic is also the best in the series by far. Being able to pick your path to avoid fighting battles you don't want (Example, if you're bad against DK and DK is on the pathway, you can just choose another path) with obvious difficulty differences (Plus the HOME Menu cheat) makes getting most of the challenges and custom parts done fun and easy.
For Glory is usually a crazy experience, ranging from Spamming Links, Ness' and Zeldas, crazy-good Falcons, brain-dead Little Macs and Charizards, and lag-switch abusers. But it's a good time if you're willing to put up with it.
Yadda yadda, etcetera etcetera, inka dinka doo, you get the jist.
Smash 3DS is good. You should get it.
And after that, get Animal Crossing: New Leaf. It's good.