Mario vs Donkey Kong March of the Minis is great.

I forgot about Trauma Center, great recommendations everyone, I wasn't expecting so many good games I need to write down. And yes I want a 3DS right now, I would play the crap outta that 3d Pokemon.

SMT4 would be good...

O.o

Hmm, interesting.

Hidden gems? Custom Robo Arena. Available in many languages.

Does look good.

Manhattan Requiem. A great detective adventure game I'm playing now. :)

Is there an English translation? It looks like an interesting game, but doing Google searches only brings up the Japanese version of it.

I've not find any ones. unfortunately.
Even this game sets in new york city. buh!

Blast it!
Curse you Japanese with your wonderfully-crafted Japan-exclusive games!
*shakes fist*

since you're interested in Lufia nds
Many have trouble when using a character named Guy because he is slow and it's hard to connect his combo that seemingly to start with a slow bing swing.

He is actually as easy as any other character in the game to use. People who have hard time using it mistook his big swing as his first attack. There is actually a weak kick before that swing. If that kick connects, the rest of combo will connect. Keep that in mind if you decide to play it

Also, the game is not very optimized so in some dungeon you'll experience slowdown. Nothing is game breaking but I think you should know

Beware: it's kinda rare, and consequently kinda expensive.

Contra 4 is another title worth mentioning, as is Shiren the Wanderer.

Don't it's a trap, Dark Spire is so difficult/grindy at the start that I couldn't be bothered even though I love te Etrian games.

Metroid Prime: Hunters
All Castlevania games
Contra 4
GTA Chinatown Wars
All Phoenix Wright gamers (Including Apollo Justice and Miles Edgeworth ones)
The World Ends With You
Mario & Luigi games
Kingdom Hearts 358/2 Days
Megaman ZX games and Zero Collection
Hotel Dusk Room 215
Digimon World DS and Dawn/Dusk
Ghost Trick
Elite Beat Agents

Everyone already listed the best games on the system. I'll just add a few :

Pokemon Mystery Dungeon Explorers of Sky : Great D-RPG with an awesome story that will make you cry

Pokemon Ranger Night Over Almia : Fun Spin-off with great music and fun characters.

Okami Den : The sequel to Okami that is better in every way (NO ISSUN !!!)

Radiant Historia : Probably the best RPG on the DS with TWEWY. The graphics are not that great but the story, the characters and the musics are awesome.

Valkyrie Profile Covenant of the Plume : My favourite game in the serie. A hard T-RPG where you control a human who wants to kill the Valkyrie. Really fun with a great story that has multiple endings.

Final Fantasy XII Revenant Wings : A Real Time Strategy game that is the sequel to FF XII. It uses a "paper - rock -scissors" system during battles kinda like Fire Emblem. Pretty fun with great music and better developped characters

Knights in the Nightmare : This one is kinda hard to explain but it's an interesting game. It's a mix between a stratey game and a Shoot them up.
On the battlefield you control a wisp that can possess soldiers by giving them weapons, while dodging ennemy bullets.
The tutorial contains many info but it's easy to learn and the game is a blast to play

Dammit, people have said all the good ones.

Well there's also:

Henry Hatsworth and the Puzzling Adventure (Unique Puzzle/Platformer game)

Final Fantasy Tactics A2: Grimiore of the rift (Good turn based strategy game)

Any of the 2nd series of the Pokémon Mystery Dungeon games (Time/Darkness/Sky) (Another turn based rpg game)

Pokémon Conquest (The Nobunaga's Ambition and Pokémon Crossover, turn based strategy game)

Legend of Zelda: Spirit Tracks (Great little Zelda Game)

Trackmania DS (Racer game where you can also build your own tracks)

Others that I don't own but can think of off the top of my head are:

GTA: Chinatown Wars

Anno 1701: Dawn of Discovery

Age of Mythologies

Scribblenauts

Picross DS