I have every RPG on the 360 and PS3. The one I liked least was Blue Dragon. Annoying main character, bland monsters, and really really bad turn based fighting because you transform into these blue monsters that basically punch the enemies most of the time. Don't know if it's the worst RPG. I just didn't like it as much as the other RPGs and JRPGs.
I would say Last Rebellion was the worst, but I'll be honest when I say I didn't last more than half an hour or so after starting it up, so I might be just a bit biased. I abhorred that game - I hate it with such a seething passion that I can't be objective with it at all. Like, everything in ti turned me off from it - the battle system, the characters, the world, and the art style.