1. Castlevania: Dawn of Sorrow. This was my first DS game, and even after beating it once, i found myself going back to it and replaying it again and again. The upgradable weapons and collecting enemies' souls gets quite addicting, and the extra modes included are top-notch. Well worthy of the top spot on my list.
2. Picross DS. I bought this in August 2007 and i played it regularly every day until about October, when i had beaten most of the puzzles and spent tens of hours trying to finish them. Better yet, there are new puzzle packs to download as well as daily challenges, so Picross is sure to keep you busy long after you complete the main puzzles.
3. Lego Star Wars: The Complete Saga. It really tells you something when i finish the 30 levels spanning the six Star Wars movies and only collect 52% of everything there is to find. The value you get with this game is fantastic, as you'll be playing through the story atleast twice and then searching every nook and cranny looking for that final minikit piece. Great value.