Dragon Quest IX: Defenders of Starry Skies - Good RPG, with lots of classes and skill-sets and the unique, original town-by-town subplots the series has been known for since the 3rd game. Also, the DQ IV - VI remakes.
Shin Megami Tensei: Strange Journey - Addictive sci-fi dungeon crawler with tons of demons to fuse and collect, an atmospheric, involved plot-line and challenging core mechanics.
SMT: Devil Survivor - It's strategy. It's SMT. It's got a great plot.
Pokemon Black / White Version - Do I need to explain? It's Pokemon. It's new. Get Platinum too, it's the only one where you can raise Houndour before the elite 4 without trading.
Suikoden Tierkreis - Not as good as, well, any other Suikoden game, but it's good in its own right. Hundreds of characters, well designed dungeons and a unique storyline make it a decent addition to the DS library.
Final Fantasy Tactics A2: Grimoire of the Rift - If you like the gameplay and class system from Final Fantasy Tactics and Tactics Advance, this is pretty much the perfection of the formula. If you're looking for the plot, then this is a lot more involved than the original Tactics Advance but ultimately still falls short of Advance's plot and especially Tactics' amazing one.
Final Fantasy IV - The definitive version of the game, augments and extra scenes alone really make this a lot better, and the graphics are an improvement upon III. A lot harder than other versions.
The World Ends With You - Best new Square game in ages. The story, characters and gameplay will hook you, and leveling up pins is oh-so-satisfying.
Knights in the Nightmare - A fresh take on SRPGs by combining them with bullet hell, it's a bit vague and simplistic at times but it works, and the soundtrack's great. Think Riviera without all the 13 year olds' sex fantasies.
Mario and Luigi: Bowser's Inside Story - Once again, shouldn't have to explain. It's Mario. And Luigi. They kick ass in Bowser's stomach while Bowser fucks shit up.
All I can think of right now, I'm sure you've played a few.
