In no specific order:
1. The World Ends With You
A gem of a RPG, this is just amazing. When I first heard about this, I thought that the concept was just stupid and that this will end up to be one of SE's failures. I decided to pick up the Japanese version about a week before the EU version came out just to test it and it got me hooked. Massive customization, awesome battle system, high replay value, definitely one of the DS' best.
2. Final Fantasy CC: Ring of Fates
I loved the first game because of its fresh approach to the FF world, despite all the criticism. However, Ring of Fates is definitely a improvement from the first. Single player is actually pretty good, even with the stupid AI characters and somewhat repetitive gameplay. However, the multi-player mode is where the game really shines. Playing with friends is much more enjoyable than those AI characters.
3. Pokemon Diamond and Pearl
This is definitely overrated, but it's still one of my faves. At first glance, it looks unimpressive. It seems to be just another monster RPG, graphics that aren't exactly something to write home about, and a basic interface/gameplay. Actually, the massive amount of depth in this game is astounding! After all, this is probably the only RPG to have a hidden IV/EV system to allow you to "customize" your monsters (to an extent). And besides, millions of people can't be wrong... or can they?
4. Metroid Prime Hunters
I won't lie, I completely suck at this game but even a crappy gamer like me can tell how amazing this game is. This is the best FPS for the DS so far. Sure, the single player mode is really easy and short but the . 7 characters + 7 alt forms + 8 weapons (I think) + an almost flawless 3D engine (except for a few glitches here and there) in multiplayer mode makes up for it. And don't forget about the outstanding Wifi modes!
5. Mario Kart DS
This is without a doubt the best racer for the DS so far. When this first launched, it brought about a new innovation for the DS - online Wifi races. I used to spend so much time on this trying to get triple star on all missions, unlocking all the staff ghosts, etc. I've accomplished all that now, and I did it all without snaking. =]
Honorable mentions:
LoZ
H - excellent use of stylus, very innovative for a Zelda game
Advance Wars Days of Ruin - a completely new Advance Wars game, fixes most of the errors that the previous had
Ace Attorney series - great storylines, cool characters, gives you a sense of accomplishment when you complete a trial
...Holy crap, that took a long time. =/