Ah, the immortal game where the Megaman series became its own franchise. With the new inclusions of the password system, energy tanks, items (such as the jet sled) as well as weapons that have more of a purpose than killing things (Crash Bomber and Time Stopper come to mind). It also implemented a final boss that can only be harmed by one weapon, and a control system that was very close to being refined.
This game rates equal to Megaman 5 in quality of music; the songs are just so catchy and good. And you'll have just as much fun playing it as listening to the music. Some people claim that this is the easiest in the series, but I say that a game is only as easy as you make it. Just because you can beat an entire stage using the Metal Blade and not run out of weapon energy doesn't mean that you have to use it. This game is great in that you can make your own challenges and tackle situations in any number of ways. This is a must-own for Mega Man fans and NES buffs alike.