New Super Mario Bros. DS
This is classic side-scrolling Mario Bros action, a successor to the original Super Mario games. The new moves in this one include a super-mushroom that turns Mario into a giant. Mario's adventure spans both screens.
Mario & Luigi: Bowser's Inside Story
You could start your DS collection with New Super Mario Bros., which is as fundamental as it gets, but try this twist inside. Mario & Luigi: Bowser's Inside Story is the newest in the series of creative role-playing games starring Nintendo's famous brothers and their wonderful cast of friends and enemies. This game has exploration, tons of inside references to past Mario games, more of the series' top-flight comedy (this is a rare game that will intentionally make you laugh), plus, it's got a twist: You play as Bowser in the top screen and you play as Mario or Luigi — who have been swallowed by Bowser — in the lower screen, exploring his, uh, guts. Read Kotaku's Mario & Luigi: Bowser's Inside Story review.