I don't really have a favorite...
Honestly, I think the most memorable ds game I played is Contact for the ds, which isn't fully a fun game, but it has a lot of heart in it's art, and the music is top notch (give it a listen!) In terms of the 3ds, the game that I would say is memorable is Tomodachi Life, the game gets boring fast after your second or third restart, but that first one is the most memorable.