Shadow Dancer is one of the best old school ninja games you can get your hands on, its only real competition coming from Shinobi III. It plays similar to other Shinobi games, though the action seems a lot faster. In addition, things are given a new twist in that you are accompanied by a faithful dog who will attack your opponents on command, resulting in a struggle between the dog and your enemy that gives you time to take the bad guy out. This move is very useful for opponents who shoot at you, because they can't shoot while your dog is busy tearing off their arm. You also have a dose of ninja magic that you can call upon once per stage, and using it will take out every enemy that's on the screen (makes a great panic button). Finally, at the end of each level, a big scary boss awaits you.
Despite the fact that it for some reason often gets a bad rap, Shadow Dancer is probably my favorite Shinobi game.