People are probably recommending the obvious stuff like Dragon Quest and Castelvania, so I'll throw out a couple of underlooked gems.
- Professor Layton and the Unwound Future
The very best Layton games, with smart puzzles, fun mini-games, and a surprisingly sweet, sad story.
- Henry Hatsworth in the Puzzling Adventure
An incredibly inventive, overlooked platformer game with solid combat, light Metroid elements, and some puzzle elements.
- Tetris DS
The best version of Tetris, imo, with tons of modes, including some satisfying puzzle modes.
What about Kirby Canvas Curse? It's a fun platformer where you draw lines with the stylus to steer Kirby through the course. Gets pretty challenging! One of my favorite DS games.
I don't even like Kirby games, but Canvas Curse is incredible.