The best DS game bar none, but..
A predisposition towards Yu-Gi-Oh! the Card Game is require to enjoy this wonderful treat.
Pretty much for the fans, World Championship 2007 is beyond wonderful, despite it's flaws. It has the most comprehensive card database, semi-impressive graphics, but what ultimately shines here, is the measure of replayability. Virtually endless, you could play this game for a very long time and not hit the end, plus it comes with online mulitplayer.
If you were looking for a game that finally grabs the spirit of the card game, than look no further. Please be aware that this is 2007, and not the 2008 version of World Championship.