My favorite combat system is by far Ninja Gaiden 3 Razor's Edge despite its flaws in general. Having played NG:Black, Sigma, 2 and Sigma 2 at this point, have put a lot of time in each system, i still keep to this thought. NGBlack/Sigma being my favorites after and the two versions of 2 being my least favorite games in the series (i think i even enjoy Ninja Gaiden: Dragon Sword over them). I think the additions to ryu's moveset, while some key changes in enemy design and health, even though there was a lot of negative changes with enemy hit reactions and stun, makes the game more interesting to me.
Would love Ninja Gaiden 4, but is not happening this gen at all.