I agree. The DS is an amazing system and I've still yet to get more games from it!There are so many excellent titles on this little beast, it's hard just to pick one. The Nintendo DS is sort of the PlayStation 2 of handhelds. It's a system which has achieved incredible sales numbers with wide support from developers, ensuring a slew of amazing games.
My all time favorite DS game is probably Contra 4 developed by WayForward Technologies and published by Konami. I absolutely love the Contra franchise so this was a day one purchase for me. Fortunately, I wasn't disappointed with my purchase. This is a game that's got it all: awesome pixel art graphics, brilliant soundtrack and non-stop shooting & platforming action. The controls are tight and the end of level bosses are awe-inspiring. It's also a game that I always go back and play, it's that addictive.