The one and only action game.
1. NG II has the most badass character of all time. Ryu Hayabusa is somewhat like the teched down version of batman, but much more ruthless. In short: he is the master of cutting enemy into pieces.
2. NG II has the best control feeling / melee physics of all time. This is very subjective, but I've tried God of war, Devil may cry, Prince of persia, and many more other action games, none of them come even close to NG II in terms of phycially plausible melee control feelings.
3. NG II has awesome graphics and atomersphere. A mixture of japanese folklore and western myths, the art style is very refreshing and pleastant to watch.
4. NG II is so difficult that you "feel your dick extended several inches once you beat the Master Ninja mode" (quote).
BTW, from what I've seen so far, I don't think Ninja Gaiden Sigma 2 will have a chance to compete with NGII. That game is more like "Nude Gaiden Sissy 2" to me.