Well, I beat Castlevania: Dawn of Sorrow for the Nintendo DS.

I dare say, that this is probably the best single player game out for the DS at the time I am typing this.  I really wanted an RPG, but after hearing about Lunar: Dragon Song and not really careing about another Mario-type RPG (which aren't really as fun as real RPG's).  I decided on Castlevania which was action/adventure with RPG stats as you gained XP from defeating the enemies and could also shop for equipment with the money you earned from kills (although only potions are worth buying).

What makes this game unique is the replayability features it offers (btw I beat Julius Mode) and the fact you have the potential of gaining the soul of enemy you defeat in the game and gaining that NPC's ability.  You will always get bosses soul's as they usually posses an ability that allows you access to the next part of the castle.  Also, the whole game is takes place in a castle-like place so if you miss something you can always go back for it, or take the transporters for quick access to certain parts of the castle.