Contra

Contra is very close to a beat-em-up and a FPS game combined.  The first level is a side-scroller.  As you move through the game you gain upgrades to your weapon.  It has extremely hard bosses and in general is a very tough game to play. 

The characters look like your ordinary tough guys.  Konami used the big action stars and movies of the day to create a game they believed the people wanted to play.  It is almost too hard to play.  Without the use of the Konami Code it is pretty much unbeatable.