Well, Bros. In Arms has stayed on my DS since I got it, and I've already played through every mission on every difficulty level. Just a great game, warts & all.
MKDS, of course...just `cause.
Puzzle Quest...of course. Everyone probably has that one.
NSMB has recently become a mainstay, just for when I'm in the mood.
And interestingly enough, I've yet to get rid of Kira Kira Music Hour. It's my favorite of all the rhythm games. There's a ton to do and unlock, unlike EBA or most others.
I've tried just about everything else that's come down the pike, and some games have been great, like Joker and Revenant Wings. But once I'm done, I'm done. However, Phantom Hourglass is still on my card. I just can't bring myself to erase it, even after playing through it twice and starting a third time.