One of the most popular series through the history of video games is the Mega Man series. The games in this series streches from the NES to the SNES to the N64. But, like every series, that has to be a start somewhere, and the NES game Mega Man is where this series all began.
Mega Man introduced an interesting concept to the gaming world. Instead of playing the game in the order of pre-sequenced levels, you get to chose whichever order you wish to play the levels. Starting in this game with six levels to chose from, eventually evolving into eight levels in later games, the original Mega Man is definitely the most primitive of the series. It's also recognized as one of the toughest in the series.