To the person who asked if RE:DS is any good, yes, it definetely is. It's pretty much a direct port of the PSX version but with the ability to use the stylus as well as an extra minigame. Hearing Barry's bad voice acting 10 years later fills my heart with glee. : )
As for Final Fantasy, same deal. The graphics are a bit on the klunky side and take some getting used to, but the game is just really easy to get sucked into.
Trauma Center, as someone mentioned, is one of the most inovative games I've played in years. It IS incredibly difficult though, but I'd say it's worth it.
Elite Beat Agents is a must if you enjoy rhythm-based games. It's fun, quirky style makes it enjoyable and the increasing difficulty levels make you want to come back for more. Plus, where else can you see an overweight ninja sneak into a building to "Canned Heat"?
Castlevania: Portrait of Ruin is awesome, simply put. You obviously know the deal with Castlevania games, but this one adds in the ability to play with a friend over WiFi. Pretty sweet, if I do say so myself.
One game that you can pick up cheap that I personally enjoy is Viewtiful Joe (I forget the full name). It's an early DS game so it may not appeal to a lot of people, but I thought it was really fun.
Lastly, if you like point-and-click games, pick up both Phoenix Wright 1 and 2 as well as Hotel Dusk: Room 215. Both offer a gaming style you won't likely find in most games in the US (or UK, for that matter).