If you like platformers than any Mario game like Hooahguy said. Yoshi's Island 2, New Super Mario Bros., even Super Princess Peach.
However, my favorite games are the Mario & Luigi RPG series. Partners in Time and Bowser's Inside Story especially (Superstar Saga for advance is also good). The Zelda games for the DS also rock. These will definitely keep you entertained for a long time.
Grand Theft Auto Chinatown Wars is one people say is good but to be honest I had a really hard time with the game being on the DS. The graphics are too small and not great (they just don't work) and the controls are unwieldy.
Just check out some of these on Youtube and see if you like.
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