Pokemon
New Super Mario Bros. is a rent, not a buy, to be honest. Didn't last long enough to engage me. Same goes for Kirby: Squeek Squad. Awesome games, but not keepers.
Final Fantasy Tactics A2. I haven't played it. but if it's anything like Advance, you won't be sorry for buying it. It's similar to Disgaea.
Of course, there's the Ace Attorney series, which are just excellent.
If you didn't play it the first time around, the DS port of Super Mario 64 is good. Not as good as the N64 version, but still rather good.
There's also a DS port for Chrono Trigger coming soon/already out (depends on where you are). If you like JRPGs I cannot recommend this game enough.
Oh, and if you liked Advance Wars, there's a DS port of Fire Emblem (the first one, the Japanese one that never saw release outside Japan up till now) coming soon. In a similar vein, Final Fantasy III is out worldwide and available on the DS... hell, there's a lot of Final Fantasy games on DS. Not to mention other SquareEnix games.