since DSLover loves to resurrect dead threads, I'll highlight a couple games that i've liked, that aren't the "Must have" games.
Mario vs. Donkey Kong 2: March of the Minies - Sorta like lemmings, but totally not. Excellent use of the stylus to control your Marios. Also, varying levels of difficulty, and eases you into the game very well.
Lunar Knights - Hack and slash RPG. You control two vampire hunters, one who uses dark swords, and the other uses solar guns. So far, very entertaining, and ~10-15 hours of game play.