Listmania!
Must-Own Nintendo DS Games
By an Amazon.com customer
Star Fox CommandStar Fox Command
Buy new: $18.50 / Used from: $4.58
A worthy addition to the Starfox franchise. I've played each Starfox since the N64, and this is the BEST. The only problem is you can't choose your character when playing online.
Metroid Prime HuntersMetroid Prime Hunters
Buy new: $82.98 / Used from: $5.00
A contender in the FPS genre. Samus Aran returns for a kick-butt adventure. Multiplayer far outlasts the single-player.
Elite Beat AgentsElite Beat Agents
Buy new: $10.50 / Used from: $3.45
A fun deviation from the typical fighting or action game. Has somewhat of a learning curve.
Planet Puzzle LeaguePlanet Puzzle League
Buy new: $29.99 / Used from: $21.00
The best puzzle game on Nintendo DS (except Tetris). A variety of modes and online play means you will come back to this game for a while.
Tetris DSTetris DS
Buy used from: $28.58
It's Tetris. On the DS. What more is there to say?
New Super Mario Bros.New Super Mario Bros.
Buy new: $31.99 / Used from: $17.98
A nice return to form for the Mario platformer franchise. Play as either Mario or Luigi. High replay value.
Clubhouse GamesClubhouse Games
Buy new: $19.49 / Used from: $9.48
A great pick-up-and-play collection of card, board, and variety games. Multiplayer owns single-player here.
Mario Kart DSMario Kart DS
Buy new: $33.49 / Used from: $19.49
A standout in the racing genre. The controls are sharp, and it looks decent. Multiplayer and online play are worth it.
Race Driver: Create & RaceRace Driver: Create & Race
Buy new: $32.99 / Used from: $10.80
An interesting title. Create your own tracks, then race against AI or friends or strangers. If you're not into racing with strangers, you can post the custom tracks online (I think).
The Legend of Zelda:  Phantom HourglassThe Legend of Zelda: Phantom Hourglass
Buy new: $32.49 / Used from: $14.99
One of the better adventure games. Ingenious use of the touchscreen.
Advance Wars: Dual StrikeAdvance Wars: Dual Strike
Buy new: $74.27 / Used from: $14.65
A nice strategy game. Multiplayer is great, even better when your friend owns the game. It gets intense!
Animal Crossing: Wild WorldAnimal Crossing: Wild World
Buy new: $33.49 / Used from: $15.25
An alternative to Second Life. It keeps playing! Very good customization of your house, clothes, etc.
Lunar Knights: Vampire HuntersLunar Knights: Vampire Hunters
Buy new: $57.44 / Used from: $11.24
An overlooked gem. It reminds you of the dungeon crawlers of yore...
Custom Robo ArenaCustom Robo Arena
Buy new: $13.75 / Used from: $6.15
An excellent fighter. Story mode takes a while, but the multiplayer and the customization are worth it.
Viewtiful Joe Double TroubleViewtiful Joe Double Trouble
Buy new: $19.99 / Used from: $5.12
Another overlooked gem. This time, it's a side-scrolling beat 'em up. Creative use of VFX powers.