Side-scrolling shooters don't get more pure than Contra - and for good reason too - as even though in recent years the series has all but become dead and buried, should Konami ever decide to release a modern-day version the amount of fans who still swear by the original game are everywhere. You need only take one look at the latest screens for Metal Gear Solid to get an idea of just how proficient Konami's team always is with console hardware, and even back when the series' made the jump to the Genesis, Hard Ops would show off everything from more detailed explosions and energy weapon-fire, to screen-sized bosses and slick animation. Chances are whichever your first Contra was will be the one you cite as the hardest, as like the other entries on this list, it's in those crucial few opening levels when we're all scrabbling to get used to a game's controls that the initial buildup of hatred settles in.
