I've played and finished every rpg on the 360 (bar Mass Effect which I'm working through).
I'd rate them in the following order:
JRPG:
Blue Dragon ~ Great game play, looks great and plenty to do (A)
Enchanted Arms ~ Good story, good characters, looks pretty good, not a bad jrpg (C+)
Eternal Sonata ~ Beautiful to look at and interesting story but repetitive combat, and poor dungeon design (C-)
Phantasy Star Universe ~ Fun story but a bit graphicaly dated and can get a bit tiresome towards the end (D)
RPG:
Mass Effect ~ Amazing, ya' know the rest. (A)
Oblivion ~ Super, vast, great open world with lots to do. Main quest could be better and Shivering Isles (goty edition) is top notch. (A)
Two Worlds ~ Better than expected, if you're willing to put some time in and don't mind a more traditional rpg with a bit of a dodgy engine you'll love it. (B-)
There's also Marvel: Ultimate Alliance which is more a hack/slash rpg but still great fun (espcecially coop).
If you want a jrpg get Blue Dragon first.