What is your favorite DS game? Don't just name a game, be sure to tell why! 
I'd go with Mario 64 DS hands down because Mario 64 is one of my favorite games ever made. It's so immersive, and was one of the first solid platform games released. The DS version is better than the N64 version by far, and aside from minor gripes (such as making some stars easier), it's easily my favorite DS game.
I should mention that it's the only DS game I own, and the only DS game I've played, so I'm a bit biased on this subject. It's single-handedly the only reason I bought a Nintendo DS.
I'd go with Mario 64 DS hands down because Mario 64 is one of my favorite games ever made. It's so immersive, and was one of the first solid platform games released. The DS version is better than the N64 version by far, and aside from minor gripes (such as making some stars easier), it's easily my favorite DS game.
I should mention that it's the only DS game I own, and the only DS game I've played, so I'm a bit biased on this subject. It's single-handedly the only reason I bought a Nintendo DS.