Mario Kart DS (WiFi is fun) New Super Mario Bros. Yoshi's Island DS (playing this one right now, really fun) Lunar Knights (Action RPG made by Kojima & Co., really good replay value) Kirby Canvas Curse (I want this game, but I can't find it anywhere) There's also the new Pokemon games coming out if you're interested. Those'll probably be the biggest DS games to date.
Mario Kart DS is a must have. I'm getting Pokemon Diamond on Sunday if you're interested in that kind of stuff. Castlevania's are amazing on the DS. -Magical Samurai
Mario Kart DS Pokémon: Diamond/Pearl New Super Mario Bros. Elite Beat Agents Castlevania: Portrait of Ruin Lunar Nights Super Mario Bros.: Partners in Time and give Eragon a shot. Yes it's licensed, but it really is a great game. I thoroughly enjoyed it. Those games are just the cream of the crop; the must-buys. There are plenty of games that aren't as fantastic but still very good.
I highly recommend the Castlevania games on the DS, Dawn of Sorrow and Portrait of Ruin. But before you get Dawn of Sorrow, you should get Aria of Sorrow on GBA for the storyline.
Mario Kart Mario 64 Super Mario Platformer 3D Starfox (Haven't tried, but I am buying this weekend Possibly Pokemon (The last one I bought was like red version, but I think I might start up again since it's wi-fi, haha I am buying this weekend also)