Contra is widely accepted as the most difficult game ever designed and if you claim to have beaten it without the famed code, well, you sir, are a liar. Of course, the beloved run-and-gun masterpiece from the NES era made the cut!
Although Contra was released in the arcades in 1987, it wasn't until 1988 when the game made its way into people's living rooms on the NES that the game truly made an impact. On the biggest selling points for Contra was the two player co-op, which was rare for the late 80s.
Top 100 Video Games of All Time #60 - Contra