Ah hotel dusk... i was trying to remember the name of that one too.

 what about Contact, or Lost in Blue? would anyone reccomend them? (i think LiB s meant to have a slow start but it gets better)

some more:
Bomberman if you have 7 friends to play against.
42 all time classics (assuming you like card games/billiards/darts and such)
Project Rub?
Worms open warfare.

Bomberman Land Touch! has an online battle mode with Wi-Fi Connection.

@TWRoO:

I have to say, I totally love your avatar and signature. Black Adder is my second-favorite television show all-time.

applause?

Avoid FF 3. It is a really bad game. It was boring enough to make me give up near the end of the game.

Since nobody else has suggested it, Pokemon Diamond/Pearl.

I'd say Brain Training and/or More Brain Training. Big Brain Academy is also fun, but it's more "traditional" puzzle game than Brain Trainings.
And of course all the games mentioned in this thread.

Biggest problem with DS is, that there's just too many great games to choose from.

Advance Wars rocks baby, get it, you will love it(unless u hate stat games.)

FF3 is awesome as long as you like old school JRPGs. You just have to understand that it retains the game mechanics of the NES original. I personally loved it.

For other games, definitely try Phoenix Wright and Brain Age. Brain Age may be casual or whatever, but it's also an awesome piece of software. Dragon Quest Rocket Slime is also an excellent game if you can find a copy. It sold poorly, but it's a really fun little action-RPG. Seriously underrated.

@TWRoO:
Lost in Blue is great. I think it got a bit too complicated towards the end (might just be me, but I don't think it needed so many block puzzle type things). Either way, it's a really unique game. I couldn't put it down when I first started playing. It is a niche title though and I can see some people not enjoying it, so I'm slightly hesitant to recommend it overall. On the other hand, if it sounds like something you're interested in at all, you most likely won't be disappointed, since it really is a good, high quality title.

You guys prefer Brain age over Big Brain Academy and it's crazy wifi multiplay?

Here's what I recommend:

Castlevania: Dawn of Sorrow
Castlevania: Portrait of Ruin
Contra 4
Hotel Dusk: Room 215
Kirby Canvas Curse
Mario & Luigi: Partners in Time
Picross DS
Wario Ware: Touched!
Mario Kart DS
Advance Wars
Big Brain Academy
Clubhouse games
Bomberland Touch (for the multiplayer)
Metroid Prime Hunters

What is your favourite?... I can't say I could put mine in order... but Blackadder is one of those timeless comedies like Red Dwarf and what The Green Wing will probably be. @slacker. Thanks... it looks an interesting game, but I don't have a DS unfortunately... I am just considering buying one which is why I suggested some games [basically most of what I suggested would become my DS library (plus a few others had already suggested)