A single game that makes the DS worth owning.
Rather than get into the story, I'll simply say that it's engaging and capable of pulling in someone who knows nothing about the previous Zelda games. The story-telling is a bit elementary at times, but that seems to add a level of charm you eventually come to expect.
The real strength of this game is in how you manipulate the screens and the DS itself to accomplish your tasks.
Zelda's best feature is that it makes full use of everything the DS has to offer. You don't simply use the DS to control the character's movement and inventory options. You are required to manipulate the system itself to solve some puzzles or move on to another part of the game. At times, it's embarrassing, but you are completely involved in the game in this way. To be honest, this is a more entertaining gaming invention than the motion-based controllers for the Wii.
If you're on the fence about buying a DS and have never seen this game in action, you've got all the excuse you need. It's a virtual lock that once you complete this game, you'll only feel saddened that it's over - and happy for the experience.