It all depends on your taste and what flavour of game you're after:
Animal Crossing - it's a good waste of time and has some pretty good gameplay but is a long game that you can dip in and out. Recommended
Spiderman 3 - Solid action game with little depth. Good touchscreen use
Sonic Rush - Excellent, fast platformer, more than a nod to the orginal.Highly recommended
Hotel Dusk - Engrossing story but no action. I am enjoying this one.
Elite Beat Agents - Unusual this one, weird great sense of humor but replay value will wear thin after a while. Excellent upto that point
Mario Kart - Good, in fact very very good. Highly Recommended
Mario 64 DS - Slightly dodgy controls but looks great, a solid 3d platformer
Kawashima Brain Training - Alright and only alright
Mario vs DKII (March of the Minis) - Excellent if short lived touchscreen mini classic. Recommended
Yoshis Island - Retro platformer with some good depth, quite tough in places. Recommended
Trauma Center - This is too long winded in places and rock hard to boot! Not for me certainly
Warioware Touched! - Good for 10 min bursts but pricey for what it actually is (minigames) - Fantastic touch screen demo tho!
That's about the extent of what I own - some classics and some stinkers but overall hope it helped!!