Mega Man
Mega Man is a robot classic who has fought Dr. Wily since the early eighties, and continues to appear today to fight for the good of robots everywhere. The games still look and feel great, even on the 8-bit NES. I have played the ones below, and though 5 has been out of my reach, the ones that I have played are great, and very replayable.

Mega man faces off with six robot foes, trying to defeat Dr. Wily. The foes he faces are:

Cutman
Bombman
Iceman
Fireman
Gutsman
Elecman

Mega Man 2 is probably the most classic mega man game. I remember hanging upside down playing this game... i think I'm still dizzy.

The Baddies you'll face (this is a good order to fight them in):

Bubbleman
Airman
Heatman
Metalman
Woodman
Flashman
Quickman
Crashman

It actually became funny to me - how many different man names could the developers come up with? This game is very fun, especially with the twisty plot line, and the mysterious Breakman - who will win out?

Breakman
Hardman
Topman
Gemniman
Magnetman
Needleman
Shadowman
Snakeman
Sparkman
The characters are getting repetive by this point, but new skills and mysteries help keep mega man alive.
Yaaawwwnnn... another megaman? This one is very tough too...