Late to the party, but if you're into RPGs, well...
On DS, there's remakes of Dragon Quests 4, 5, 6, and Final Fantasy 3 and 4. All highly recommended.
Chrono Trigger got a great port, too.
Dragon Quest 9 is great, too.
Nostalgia, Glory of Heracles, Golden Sun: Dark Dawn, Lufia: Curse of the Sinistrals, Final Fantasy Tactics A2, FF Crystal Chronicles: Rings of Fate, FF The Four Heroes of Light are all great, too.
On the 3DS side there are Bravely Default and Bravely Second, multiple Etrian Odyssey games, Ever Oasis, Fantasy Life, and several Fire Emblem games.
Also several Mario & Luigi games on both DS and 3DS.
I'm a bit of an RPG nut.