-Mario Kart DS
-Phoenix Wright
-Lunar Knights
-Diamond/Pearl
-Elite Beat Agents
-Phantom of the Hourglass
The bold ones are the ones that I've played/owned. For Mario Kart DS, although it's a good game, I think it's a bit too late to play it. All the good players have left Wifi and now it's filled with hackers and snakers.
Lunar Knights is a wonderful rpg that I recommend. It can get a bit 'button smashy' at times, but the majority of the game requires some sort of skill. The only real problem I saw was that you play with two different character that BOTH play a major part at some point, so if you don't train them equally, said parts will be pretty hard.
For Pokemon Diamond/Pearl, the old motto "if you played one, you've played them all" doesn't really fit. Although the concept is the same, this was obviously the best of all the Pokemon games. The Wifi system is great, the GTS is okay, the graphics are great for a Pokemon game, and last but not least, IT'S A POKEMON GAME! XD Who doesn't love Pokemon?
And finally, The Phantom Hourglass is also a wonderful game. The graphics are sweet for a DS game and it's pretty lengthy. The puzzles are fun, the fighting is interactive and the characters are great. They give you a short intro in the beginning so you don't HAVE to play windwaker (I didn't and I enjoyed it). The biggest set back though is this dungeon that you have to go to about 5 times in the game and it just gets longer and harder, and for the first four times, the monsters there are unbeatable. But hey, if you're in to stealthy games, go for it.
On a side note, I recommend
Pokemon Mystery Dungeon: Blue for the DS. An incredible spin-off from the usual Pokemon games and it actually has an enticing storyline. Not to mention the sequel come s out in a week!