Top Tier:
Tales of Vesperia- Best X360 JRPG, it's a fantastic action RPG that has little, if any, flaws
Lost Odyssey- Second Best X360 JRPG - classic turn based RPG by the original fathers of Final Fantasy who left to make their own company, Mistwalker
Dark Souls- Also on the PC, but fucking amazing no matter where you play it
Final Fantasy XIII-2- Fantastic JRPG, much better than 13, 13-2 is better in every possible way imagine and a really great game
The Witcher 2- Totally amazing RPG. There's also a PC vesion, but it takes a lot of horsepower to run it. X360 verion doesn't quite compare to a maxed out PC version, but is still one of the best looking X360 games.
Mid Tier:
Magna Carta 2- Really fun and often overlooked "JPRG" from a Korean studio, definitely deserves to be checked out
Blue Dragon- Pretty good JRPG - Pretty much Dragon Warrior, except more cartoony art. Also made by Mistwalker
Dragon's Dogma- Pretty good RPG from Capcom that they tried to make like a Western RPG
Resonance of Fate- Bad plot, bad script, kinda shitty graphics, totally awesome combat engine that's very unique
Low tier:
Enchanted Arms- Super simple, but enjoyable, especially since it probably costs like 5 bucks
Eternal Sonata- Slow, very very slow. Enjoyable if you can get past that.
Overlord- fun little RPG (also on PC)
Overlord 2- more of the fun little RPG (also on PC)
Might & Magic: Dark Messiah- Also on PC, but the console release is a bit different and has a class system rather than an open ended skill tree system and some other tweaks.
Severe problem games:
Infinite Undiscovery- One of the most obnoxious and bad localizations ever, some characters make it nearly unplayable. Voice acting in it is really bad too. Decent combat systems, but not enough to excuse the rest.
Star Ocean: The Last Hope(vastly inferior to the PS3 version) - X360 version of this was ruined by a terrible multidisc setup and the most insultingly botched localization of all time. Seriously, this shit is that bad, the PS3 version is maybe a top tier JRPG (you can use the original Japanese voices and menu art in the PS3 version. The dialogue text is still from the English and characters don't even remotely match what they're saying, but at least you don't have to listen to their terrible English script)
Final Fantasy XIII- Hallways, hallways, more hallways. Seriously, it's one straight hallway until the very end of the game. Decent combat system. High production values keep it from being total dogshit
The Last Remnant- Fairly fun RPG with a quirky battle system and a ton of weird mechanics, also lots of tons of missable stuff if you don't straight play with a guide and it's possible to completely screw yourself. Unfortunately, it runs like complete and utter dogshit. The FPS in battles often tanks and things come to a crawl. There's also a PC version that runs much, much better and has a couple improvements.