I only have 2 wifi DS games; Mario Kart and Tetris. I've played a bit of Tertis online but of the two I much prefer the Wifi play in Kart. Mario Kart DS is just one of those games that I find hard to get bored of. I can play it for ages and still have a load of fun. Hope MKWii does the same for me!
I would have to say a tie between Metroid Prime hunters and Animal Crossing: WW, but only AC:WW if you have alot of friends from fourms and such such as me
I have plenty of DS games that have Wifi, but getting friend codes is really difficult so I prefer Tetris DS online. Mario Kart too, but Tetris takes less time to get a game going.
Seriously, Nintendo really needs to get of friend codes. It's such a pain trying to get friend codes. I just want to be able to fill up my lists for games so I won't have problems finding friends to play with/against online.
I guess it would be Metroid Prime Hunters, I never played it online, but now I can, only on sundays tough...:\ And Pokémon, if you know how to Ev-train your pokemons and know about metagaming, you'll find it quite superb.
If someone wants to battle (in metroid) with me just let me know!
I can only play Metroid Prime Hunter DS online on sundays afternoon (from 3 PM to let's say 7 PM), if you'd like to battle with me let me know.