If you could only list one DS game, so far, as being that which encapsulates all the DS should be and is for you, then let's hear it.
Remember only list one and few lines about what and why it's so good. We may even get a #1 DS game which surprises us all....I'll kick off:
Man, this is tough, there are so many but...
Animal Crossing - Wild World
It's a non-game which allows you to dip in and out and play as much or little as you like. You have some freedom within the world and it also utilises the touchscreen functionally, nothing special but certainly make things easier.
Overall a good waste of time and decent interaction. I have not gone online yet but that may open things out that bit more as well.