Wizard of Oz was pretty fun for a breezy, easy to beat turn based RPG based on a beloved story all of us most likely know. It has some fun character art (all the classic characters with an anime style to them) and a unique trackball control scheme.
A Witch's Tale might be more of an acquired taste: a gothified anime story based on the mythos of different fairy tales and Alice in Wonderland, it's also a simplier RPG with better graphics but less intricate controls. Mileage may vary.
Umihara Kawase Shun Second Edition is in Japanese and there is Anti-Piracy in it that you will have to hex edit out, but it stands as one of the best and move inventive platformers released by a company not named Nintendo on the DS yet.
WWE Smackdown vs. Raw 2010 finally gave us D-Pad controls, making the game just about as close to the perfection that was the N64 AKI Wrestling games.
Cooking Mama 3 was more of a good thing for people who like that sort of thing. It was a bit on the easy side, though, but still enjoyable for the short time you play it.
Atelier Annie Alchemist of Sera Island is the game I've probably played the most in the past two months. A mix of turn based RPG and crafting, it's like playing my favorite parts of WoW without a monthly fee, gold spammers bugging me, and weird graphics replaced with cute anime designs. It's a bit on the short side as I managed to restart and get to the ending of a game within 8 hours, but you can continue to play in the world it sets up after the designated end of the competition the game is based around.
Lego Rock Band is the best "Guitar Hero"-like game on the DS so far, with full songs and the ability to play without additional hardware. If you have 256 megs to burn and don't mind a more G-rated tracklist full of Pink and the like, it's not a bad game to have around.
... And not to get in on the flame war, but COP was quite crap. A GTA game where you can't hit anything with your car and you have to be breathing on someone in order to shoot them. It's like playing Driv3r programmed the iPhone.