Ninja Gaiden Black
Ninja Gaiden Black is the dopest, most pimp-ass, most violent male power fantasy in the history of the medium, the best game in its series, and one of the balls-hardest action games ever made. Ninja Gaiden Black was an expansion on its forebearer, and featured tons and tons of extras: it incorporated new boss and enemy types from the Hurricane Pack expansion, included two new difficulty levels (Ninja Dog and Master Ninja), dozens of balls-hard missions, and difficulty settings that did more than just increase enemy health and damage. In particular, enemy placement would be different in every difficulty setting above normal, where on Master Ninja you were fighting the horrifying Fire Ninjas right off the bat. Some bosses were replaced by harder bosses, others were accompanied by smaller enemies (who would themselves grow more powerful on Master Ninja), on and on and on. Few games have done as much to make difficulty levels matter as much as this one did, and none have done more for the sake of making a balls-hard but completely fair player experience.
It's harder than Sigma and better than either of its sequels. Ninja Gaiden Black is the best Itagaki-related Ninja Gaiden, and I will brook no argument to the contrary. The Unlabored Flawlessness makes my argument for me.