My World My Way, I did not like this one at all.
The dark spire is very hard and is a good gam, but you better want something hard and unforgiving if you get it, because that is what it is.
The etrian games are both good, and both are tough. Luminour arc is another good one, both are turn based and have lots of voice acting.
Anyhow I think etrian oddysey and the dark spire, and maybe ff4(I don't remember it being that hard) are the harder thinking rpgs on the DS.