The Mega Man series is amazing.  It is a 2d platforming shooter that has been around forever and spawned different spinoffs and spinoffs of spinoffs like the Misadventures of Tron Bonne.  Of the original Nintendo series, however, in my opinion there is a persuasive argument that this one is the best or at least quite close.  

Megaman usually tries to stop the mischief of Dr. Wily's robots run amok, and by defeating a boss robot at the end of a level Megaman acquires their power (usually a special shot of some kind).  Knowing which bosses are weak against which special shot helps to fly through the game faster if you take the bosses down in some order that exploits the weaknesses of the boss characters who put up a greater fight.  This is a formula that has been used countless times in Megaman games, and while it does get old, the strengths of the game help it to stay fun.  

The graphics are always a high point.  Not only is the main character animated well (an energetic little cartoon dude blasting away at everything) the enemies in the levels are often cartoony or perhaps anime-like robots, such as hard hats who can only be harmed when they tilt up their hat to blast away at you and scamper around.  The levels also look gorgeous, and I think the exceptionally high quality of the graphics and attention to detail is a big part of what keeps fans coming back for all the sequels.  The boss designs are a huge part of this, too, as each of the bosses is based on a theme of some kind (magnetism, snakes... hardness...?) and as such they each have a personality and are memorable without ever saying a word.  

The music in Megaman games is usually mind-blowing.  Often they start off with some attention-grabbing intro (that is never repeated) and they feature groovy sounds, clever harmonies and if you pay attention to the music you will be impressed.  And it doesn't hurt that each boss character's level has its own unique music.  It adds a lot to the addictiveness of the game that you are forced to listen to amazing music while you play.  

While there is not tons that would compel someone to replay the game, the sheer level of quality and also the challenge of defeating bosses without exploiting their known weaknesses does add significantly to the depth.  

For story I rated Megaman 3 higher than you might expect for a formulaic megaman game.  The inclusion of the mysterious ProtoMan and the way the game slowly reveals his story makes Megaman 3's story a cut above average.  Also Dr. Wily's sneakiness and the way he is menacing by raising his eyebrows repeatedly while cruising in his UFO definitely bolsters the game's story-telling score (though there is nothing quite as deep as the plot of a wordy RPG or anything).  

I feel the Mega Man series can get in trouble when in every episode they include new stuff to the core mechanic that made it great.  I feel Mega Man 3 has the best balance of new stuff that enhances the game without cluttering out the old and detracting from the fun.  Mega Man 3 is the FIRST Megaman game to feature:

1  Mega Man's dog Rush, who is wicked cool and can assist the hero in awesome ways (i.e. rush coil to jump high, rush Marine the submarine and Rush Jet to fly)
2  A Mysterious guest character ProtoMan (what's his story??  play to find out!!)
3  The slide move (I like the idea of changing it to a dash later on, it really is more useful as a dash not a slide--sliding slows you down, not speeds you up, while running--but whether it's a dash or a slide the added speed enhances the gameplay)

Also, this was the LAST Megaman game to NOT feature:  

1)  the stupid charge move.  In the genesis batman robin game, which rules, simply not shooting is enough to charge your weapon on its next deployment, but in all the future megaman sequels (until recently, I guess someone at capcom agreed with me?) the player must hold down the shoot button to gain access to the charge shot, which is totally unnecessary and dumb in a Megaman game.  and it distracts from the music and graphics with a stupid charge flicker and charging sound effects.  Megaman is meant to blast away like crazy, not charge up shots with stupid sound effects that distract from the game!!!

There is a strong case to be made that Megaman 2 is also awesome, but Megaman 2 does NOT feature Rush and Protoman, remember, so if you aren't in love with those characters it's sort of a subjective judgment call.  And Megaman 1 is also a classic but a bit on the too hard side.  I think the charge shot sucking is a bit more objectively horrible and if you disagree you can suck a bug!!

But I digress, check out this great game!  And the others in the series if you want to verify all my comparisons.  

I've got to agree.  Definitely one of the better Megaman games