Atomic Robo Kid (Genesis / Mega Drive)

While it's tempting to look at this game as some sort of side-scrolling action title, players spend most of their time flying around, so Atomic Robo Kid feels a lot more like a shmup.  The game has a very nice art style, some huge bosses, and can be quite difficult.  Expect to die a lot with little warning, and to have to learn a lot of enemy placements in a hurry in order to progress in this game.