I like Morrowind alot but it was a pc game to start so that's out. I'd say it's a toss up between Halo and Ninja Gaiden. Both games moved ground in terms of graphics and presentation, especially Ninja Gaiden. Both games were also very challenging; I beat halo on legendary and Ninja Gaiden on normal and I'd say Ninja Gaiden etches out Halo in terms of sheer, overall hardness. Although, the Library or keye's levels in Halo could hold their ground with any boss fight in Ninja Gaiden. There's some good Xbox games I have not played but these are my picks, and if I just had to pick one it would be Halo of course cause' that's the game that put the box on the map.