Out of the games I own these are the best...
Mario Kart DS: Awesome racing game, and in my opinion, the best in the series.
Advance Wars DS: One of the best turn-based strategy games I've ever played. Tons of replay options.
Castlevania (either one): Two highlights of the Castlevania series.
Phoenix Wright: Awesome story and fun gameplay.
Meteos: Amazing puzzle game with amazing music.
Electroplankton: A fun diversion if you're a musician.
I think I would recommend Electroplankton over Jam Sessions, since Jam Sessions seems like it was designed for non-musicians (not enough chords, and you can't create your own chords or edit existing ones), while Electroplankton almost feels like a new instrument (which I like). Neither of the games are too deep, but Electroplankton held my interest a lot longer.
I also own a ton of other games, but I'm not sure I'd recommend them (Elite Beat Agents, Mario 64 DS, Starfox Command, and WarioWare Touched and are just a few of the ones I beat and never got back into).