Title says it all folks.

I'm going with:
The World Ends With You
Castlevania (all three but PoR first, OoE second, DoS third)

the 3 Castlevania games
Zelda phantom hourglass
Final Fantasy 4
Chrono trigger
...
The DS has so many quality titles

Legend of Zelda: Phantom Hourglass
GTA: Chinatown Wars
Final Fantasy III and IV
Mario Kart DS
Haven't played Chrono Trigger yet.

Contact is my absolute favorite. Very funny game. Some others:

Metroid Hunters -- this was my favorite FPS until everyone online started spending almost the entire match in their alternate / morph forms, making them hard to hit. That ruined the experience.
The World Ends With You -- Action doesn't get much more intense than this, and the style is quite good. Story was a bit too heavy though.
Castlevania -- Dawn of Sorrow was probably my favorite, but Portrait of Ruin was very good, too. I didn't like the last one though.
Mario Kart -- After the original and the GBA game, this is the next best one.
New Super Mario -- It's a new side-scrolling Mario!
Meteos -- Better than Lumines, I think.

I'll start off by saying that Hotel Dusk is an awesome game. One of the best DS games I've played yet! Too bad it gets overlooked because it's such a great game for those who stick with it (the game's hard and at times, can be frustrating).

My favorite DS games are:

• Etrian Odyssey and Etrian Odyssey II: Heroes of Lagaard. I am a fan of old school dungeon crawlers.
• Final Fantasy IV. Loved every single minute of it.
• Final Fantasy Tactics A2: Grimoire of the Rift. The races and Jobs appeals to me.
• Hotel Dusk: Room 215. A great game that sometimes get overlooked.
• Tetris DS. Yes, I am a tetris geek.
• WarioWare: Touched! Out of all of the WarioWare series, this is my number one favorite.

For me, there is one (well, 2) that are must-haves:
Osu! Tatakae! Ouendan (1 & 2).

Roughly translated to 'Go! Fight! Cheer Squad', these are rhythm games where you use your stylus to hit circular marks in time so that your team of cheerleaders (mostly male) can motivate a particular person. The music is great and so is the gameplay. One downside is that it's Japanese, but it's fairly easy to navigate the menus.

The World Ends With You by far.

Advance Wars, Zelda and Mario Bros. follow.

Mario Kart DS: A great game and still playing with my friends and gotta love R.O.B.
New Super Mario Bros.: Mario 2D, need I say more?
Electroplankton: I know there is a lot of critsim, but I like it for its unique ability to create great music and just a fun and relaxing game. Too bad if you want a copy you may have to pay $50 or more.
Castlevania: Portrait of Ruin: Another great Castlevania, I liked it better than Dawn of Sorrow.
The Legend of Zelda: Phantom Hourglass: The only Zelda game on the DS, but still fun.
Game and Watch Collection: Fun Minigames.
Mario and Luigi: Partners in Time: A generally fun RPG.

Of the games I have and played, here's a few gems.

Professor Layton and the Curious Village
New Super Mario
Ninja Town
Super Mario 64
Age of Empires Kings and Mythology

I forgot about the best Mario Kart. I see lots of people like NSMB, I thought it was pretty good, but too short and easy, but to be honest I don't think I would've liked it much if it didn't have wall-kicks.

Brain Age 2, if you were to base it on total play time and regularity.

Otherwise, NES/SNES RPG remakes, Mario Kart DS, and platformers like Yoshi's Island. The new GTA is pretty well done, too.

Phantom Hourglass
Professor Layton
Picross DS

I do enjoy Final Fantasy IV, but I prefer Western style RPG's to JRPG. I find it annoying in JRPGs where the characters are already pregenerated with class and starting stats... I like being able to create the type of character I want to play or even the entire party.

My favorites are:

-The World Ends With You (everything is awesome about it)
-Elite Beat Agents (my favorite music game)
-N+ (platforming at its' finest)
-Space Invaders Extreme (old-school joyous fun)
-Sonic Rush (fast, really fast)
-Kirby Canvas Curse (a ball of perfection)

Resident Evil Deadly Silence, Phantom Hourglass & GTA: Chinatown Wars. I really need to try Metroid Prime Hunters out. I'm sure I'd add that one to the list.

Mario Kart DS, NSMB, Super Princess Peach, Luminous Arc, Luminous Arc 2, Rhapsody, Retro Game Challenge, Tetris DS, The Legend of Kage 2, Pokémon Pearl, Lostmagic, Kirby Squeak Squad.

I think Phantom Hourglass is probably the best game I've played on DS - we both played through it here and adored it, even though I am yet to complete it (shame!) Everything about it was just quality from start to finish, and I really hope that Spirit Tracks can come up with something even better.

Viva Pinata was also fantastic, really enjoyable and the controls worked brilliantly. I'm half tempted to start a new garden from scratch with what I've learned, but then the intros are always really tedious. It's a great game though, but such a shame it didn't sell. I'm sure if Nintendo had published it, Viva Pinata would be a huge success.

Aside from that I really enjoyed Colour Cross and Rune Factory, even though they're not exactly what you'd call "classics". Sonic Rush Adventure was ace as well, much better than the first one although I completed it within a week

• GTA: Chinatown Wars
• Contra 4
• Space Invaders Extreme

Moon

In no particular order:

Final Fantasy Tactics A2
Zelda Phantom Hourglass
Castlevanias (#1 - DoS, #2 OoE, #3, PoR)
Chrono Trigger
The World Ends With You
Puzzle Quest
Animal Crossing
Final Fantasy IV
Final Fantasy XII: Revenant Wings

There are plenty of other great games on the DS too, but these were the ones that jumped out in my mind.