At the moment, it's Star Wars: KOTOR. It's not HARD, but it's not EASY...most of the stuff does'nt involve puzzles or anything, just talking to the right people, and I really like that. Also, the ability to become a Sith or a Jedi gives it more replayability, as I'll probably go for the Sith ending this time and then replay it and get the Jedi one :) Also, the level of customization is INCREDIBLE. BioWare did a bang up job on the lightsabers and all the other weapons.
I also think the combat system was unique, as it let you play at your unique pace. Wanna hit that Dark Jedi with a Master Power Attack? No prob, just hit the white button, go to Master Power Attack, hit it, and hit the white button again. Very simple, very good tool.
This HAS to be the most addictive game since Halo, and it's the best RPG to date on the Xbox as far as I'm concerned, and the best game yet for the Xbox, in my opinion. Period.