Cosmic Fantasy 3. I've had this title for well... 5+ years now, and just starting it, now that I've gone through part 1 (a couple years ago), and replayed part 2 (over the past month or so).
It's a LOT more polished than part 1 or 2. And quite a bit more complex as well. Part 1 was an easy-to-import RPG, and I'd imagine part 2 to be as well (though there's isn't much reason to import the JP version for part 2 when the US version is relatively cheap), but part 3 would definitely be a bit more challenging to those without some level of Japanese...