My RPG picks, trying to avoid the obvious ones.
Super Robot Taisen Og Saga:Endless frontier Just picked this up myself, is very awesome so far. The battle system is very unique, and hard to explain, it's almost a fighting game, it's very similar to the paper mario games, but more awesome. So far the story is pretty interesting, has Stargate-esque portals, and lots of fanservice BREASTS
Contact One of my favorite DS games, maybe you've overlooked it. It's an action RPG sort of, plays like an MMO almost. You get different costumes, which means different skill sets, an active sort of leveling up system (if you get hit your defense goes up, use swords and you sword skill gets better, etc.). Has, in my opinion, one of the best stories in a video game ever. It's a Grasshopper Studios game, with a story written by Suda 51, and great music. It's short (maybe 12 hours your first time), but It's pretty cheap used these days I think. I've played it every year since its release (So twice I guess...)
Lunar Knights ARPG, A very fun spiritual sequel to Boktai. Has two characters, a melee weapons fighter, and a gun slinger. The story is a little saturday morning, but the isometric dungeon crawling is very fun.