I never owned a DS, so I'm looking for suggestions from both platforms. I'm waiting for Z to get back into Pokemon, so unless there's another Gen that I need to play for some reason, I'm holding off. Currently own Etrian Odyssey Untold LoZ : A Link Between Worlds Bravely Default(still shipping) I'd like a good platformer/sidescrolling shooter if possible, but I'll play pretty much anything.
Animal crossing new leaf would be a good choice if you've never owned an AC game. Probably can't go wrong with Mario when it comes to platformers
Pokemon is a pretty obvious choice. X/Y is easily the best in the series since Gen 2. Moon was an interesting DS game that played similarly to old id FPS games, though I don't particularly remember it well or if it was even good. Going by the recent Nintendo Direct they're remaking it as a downloadable title soon, though, so look out for that if it interests you.