I used to play Pokemon obsessively, as you will see by my times.
Pokemon Ruby: around 170 hours
Pokemon Emerald: around 100 hours
Fire Red: around 65 hours
Crystal: around 150 hours
But the game I have played the most is...
Super Smash Bros Melee with around 300 hours.
I'm sure I've played other games a lot, I just can't remember then right now...I need sleep.
[EDIT]: bah, none of those games seem to apply to this thread because it seems to be only for DS.
I have many, many hours of Mario Kart DS. I have over 1000 Multiplayer wins (not wi-fi, just local multiplayer...I used to play with my friends all the time)