Sega Master System / Mark III / Game Gear
Recently re-released a a cheap price, this game has you guiding your shuriken-lobbing ninja up the vertically scrolling screen in an attempt to penetrate a heavily-armed fortress.
Although the graphics are rather simplistic, the action is challenging and addictive. The difficulty level is nicely pitched so you seem to get a little further into the game each time, and there are ten levels to beat.
If you're after a combat game with a difference, this is worth taking a look at.