Similar to the thread about Nintendo consoles, this one covers Nintendo handhelds (will probably make a thread combining the two to find out which one is best overall).
In the spirit of fairness, backwards compatibility and Virtual Console do not count. Also, Switch is not included in this poll.
For me, I would pick the DS because of how much I love the Gen 4 and Gen 5 Pokémon games, as well as PMD. Also, Bowser's Inside Story is one of my favorite Mario games. I have fond memories with Wild World, though not as much as NL & NH.
3DS would probably be second thanks to games like Awakening, Uprising, OoT 3D, ALBW, ACNL and Planet Robobot. Also felt that Gen 6 Pokémon wasn't too bad. It actually almost overtakes the DS in my opinion and it is pretty close.
What's your favorite?