If the certificate lasts long enough wait for Pokemon Platinum, FF Crystal Chronicles: Echoes of time or Phantasy Star Zero (U)
I played the Japanese versions of all these but it's incredibly hard to pick the best as they all have their own merit in certain areas.
Legend of Zelda: phantom houglass is also a gem along with Mario Kart and New Super Mario Bros.
I can't pick my favourite DS game tbh but I can tell you the game I spent the most time on was Pokemon Diamond, spent over 120 hours but if you would go for that you should wait for platinum as it is like Diamond + Pearl + a bunch of new features in one game and the story will last a couple hours longer.