Since Firestorm's talk of it was a little lukewarm having not played it yet, I'd definitely post in support of Lost Odyssey - as someone who plays RPGs exclusively for the plot, I thought Lost Odyssey's storytelling made it a must buy. It was interesting, too, in that via dream sequences there's some stories-in-a-story written by some Japanese short story writer I'm not particularly familiar with that are really quite remarkable, in addition to the actual game having a pretty decent, if cliche, plot. The gameplay in many ways feels like the game could have come out a decade ago, which could be a positive or negative depending on what you're looking for.
Final Fantasy IV's DS version is also a good purchase even if you've played it on another console previously - the graphic and sound upgrades are nice, but even gameplay wise it's the best combination of difficulty and extras of any of the FF4 releases, especially because of New Game+.