Well, people already mentioned the obvious, but I feel there's a game very worth mentioning missing:
The World Ends With You.
This game didn't catch my attention at first and I actually didn't want to play it because of the high immersion into the japanese modern culture and lifestyle (don't get me wrong, I love anime but I just didn't want to deal with it at the time).
Boy was I wrong? This game delivers the best your DS can handle: it's a very inovative rpg with an incredible story; you use every DS feature, stylus, mic, etc, without feeling the designers were just pushing it - let's be honest, there are games where you use these features without actually needing it (like Castlevania Dawn of Sorrow). And the music is to die for.
If you want something different, try
The World Ends With You.
Now, if you want something completely old school, I recommend
Etrian Odissey. Rembember those old pc games where you created your own party of adventures, dive into a first person view dungeon to fight monsters, terribly difficult monsters, one after the other? Like, you die all the time and can't seem to give up on the game? This is Etrian Odissey, a very good game (with an even better sequel), where you even have to map the dungeon in a grid using the stylus. Very fun, though It might bore you if don't like hard games.
Other good games:
Castlevania Portrait of Ruin. Just great. If you played through Aria of Sorrow on the GBA, then I also recommend Dawn of Sorrow, wich is a sequel. Otherwise, stick with PoR, unless you're a Vania junkie like myself (and later this month, Order of Eclesia is coming out - the hype is very strong
)
Puzzle Quest. This is a gem (pun intended), a mix of Bejeweled and RPG, where you're rewarded for being addicted.
Final Fantasy Tactics Advance 2. Though I liked the more serious storyline from the PSOne, this game is a lot of fun too.
Luminous Arc. Even simpler than FFTA2 (much less time fiddling with character optimization), but very a good tactical rpg. The story starts quite simple, but unfolds in a rewarding way. Some voice acting are annoying but the humor tone makes it okay. Also, it features the best angel I have ever EVAR seen
(comical, that is).
Dragon Quest IV Chapters of the Chosen. Remake of an old style game, very good because the original game was good. Not many new features added, so you'll feel like playing SNES or something.
Final Fantasy IV. Also a remake, but unlike Dragon Quest, completely remade with 3d characters (wich I hated, but cant really complain) and voice acting. I played it on the GBA, and it was a very good old style rpg. If you havent played, dont miss it (it's one of the most classic Final Fantasy games, along with VI, whose remake is also comming soon)
New Super Mario Bros. It's Mario. What's not to like?
Good luck!
