Ninja Gaiden Black (not Sigma)
Although the DMC series revitalized and defined the previous generation's 3rd person action genre, I feel Ninja Gaiden had the most polish. God of War was the most cinematic, and DMC was the most stylish, whereas in Ninja Gaiden it's all about timing and precision. You could effectively UT your way through the entire game if you know when to use it and can utilize it at the righ time. It doesn't really focus on pulling of stylish combos like the DMC series and to a lesser extent the GOW series, but more on being on your feet and knowing when to attack, since enemies actually want to kill you.
It also has a ton of replay value, because the difficulty setting is handled brilliantly, as the rarer enemies replace weaker enemies on higher difficulty settings, rather than the old "enemies do more damage and can absorb more damage" it actaully tests your skill and forces you to adapt, seeing the game in a completely different light.
Mission Mode, which was basically a free for all and kill everything in sight, while try to advance in a time limit, some stages are insane, but it's all the more satisfying once you beat it.
Oh and finally this game redefined hard and pushing you to your limits in patience, but it wasn't cheap hard (NGII) but was a refreshing type of hard where if you die, it's cause you fucked up rather than a cheap mechanic in the game. You could effectively make your way through the entire game without ever needing to use an elixir if you play your cards right and know all the tricks.