Here's a list of all the games available for DS that have the online capabilities.
http://en.wikipedia.org/wiki/List_of...nnection_games
So there are very many DS games already out that have online play.
At least what I know of, Mario Kart is one the best if you like fun arcade racing.
Worms is great for quick strategy battles, Metroid is good for FPS deathmatches etc.
Clubhouse Games / 42 Classics has all kinds of card games and other pub games in it.
Also Animal Crossing seems to be very good.
Castlevania is a co-op game in multiplayer mode if I remember correctly.
There are many games in the list that I don't even know about, so there might be lots of more good online games available.