4. Mega Man (1987)
The one that started it all. It's hard to write a lot about this entry since so much of it has been said in a modified form, due to most of the sequels being improved remakes of this old school classic. The robot masters, stage design, music and everything are all here in top-notch form. My only minor complaint is that this has to be one of the most difficult games in the entire series. While certainly not impossible, the later games learned how to funnel the difficulty in different areas instead of making every aspect of the game feel like a grand frustration. Still, this is an important game in the medium's history and a classic by any standard. If you haven't played through Mega Man, you're missing out on one of the most definitive games ever made.