Ninja Gaiden has pretty much everything you would want from an action game not only from its time but even from an action game you would play today. It has high production values, challenging yet satisfying combat, a great sense of variety, and refuses to get in its own way with an overly complex story.
Even the game play, while still challenging, isn't particularly difficult to wrap your mind around. It's more of a matter of training your mind and muscle memory on how to respond to certain situations then it is learning some complex control scheme to pull off certain moves.
Ninja Gaiden has it all, and it should be no surprise that it has spawned its own new series of games as well as a remastered collection that is on the way right now. Hopefully, if that does well, Team Ninja will take it a step further and give us a new game someday.