Arisato-kun said:
Tales of Vesperia is probably the best RPG on the 360. I'd say go with that. The story is interesting and has some surprising twists. It doesn't have a typical turn based battle system like most RPGs. Grinding is actually fun, though not required. Plus the grade shop and titles will keep you playing it again and again.
Really?
I'm a bit of an RPG nut but the demo didn't get me that interested...
Though i'm going to give the demo another go now you've said that.
Echoing some other people:
RPG-wise Lost Odyssey is a must IMHO
fallout 3 + oblivion (though if you have a PC that's capable of handling them, get the PC version at all costs. The mod scene is beyond amazing for these games, you're getting half the game on consoles.)
I also rate highly: Blue Dragon, Eternal Sonata, Star Ocean 4, Fable 2, Last remnant, Mass Effect. (depends what style of RPG you like though, it's a fairly random mix i've mentioned

)