FFT-style wise, Devil Survivor was pretty nice, iirc. Forget how lengthy it was, though.
Other stuff... radiant histora was good, from what I played of it.
Infinite space is pretty neat; there's a bit of an LP of it lurking somewhere back in play with your buddies.
Knights in the Nightmare is also neat, if you can/once you wrap your head around how to play it.
I personally enjoyed what I played of Black Sigil... battle system was pretty nice for what it was and the writing/plot cracked me up. Iirc a common criticism of it though is old school encounter rates, heh.
The Super Robot Wars thing (Endless Frontier) is... is. Kinda' fanservice-y, but interesting combat, fairly well written/amusing dialogue despite it, etc., etc.
If you feel like wrangling fan translation stuff, Soma Bringer is a pretty neat action RPG thing, I believe. Never got very far into myself, but still. There's actually a few other NDS games that got unofficially localized, heh. Sands of Destruction is another one on that front... again didn't get very far into it, m'self, but what I saw was intriguing. Also heard pretty good things about 7th Dragon.
Echo etrian odyssey. I actually kinda' enjoyed what I played of the ones I did play, and I kinda' despise that style of first person dungeon crawler.
That's a handful off the top of my head, anyway. Bunch more I forget at the moment (it's a quarter past midnight
) and haven't been mentioned so far, too... DS actually has a fairly killer library even without getting the 3DS involved, imo, if a fair amount of stuff that didn't get much attention despite being pretty ruddy solid games. Will say most of the stuff I didn't get very far into had a lot less to do with the games themselves than personal attention span and hardware issues. Stuff I didn't sink much time into mostly wasn't 'cause of the game's quality, heh.