Megaman 2 is clearly the best. Solid level design, great bosses and power ups and a kick ass sound track. Megaman 2 also brought us something that has been missing ever since, bosses that are effected by multiple weapons. Sure, you could use Flashman to bring down Quickman but you could also use Heatman as his power is just as effective. Most of the bosses had 3 weapons that they were weak against. Every other game has gone down to a rote rock -> paper -> scissors routine and I don't like it. It practically forces you to play the game a certain way. I like that in Megaman 2 I am more free to choose my own path and try different things and not go "I don't have the right boss power" You still might not have it but you might have something that can get you through it more easily than the power pellet.

That said, my next favorite is Megaman 5. I thought that 5 had some great level design and some fun bosses. It was nothing special but I really like the overall feel of the game.

I also really like X5 though it has been so long that I can't remember much about it other than I really liked the hell out of it.

I can't wait for Might No. 9 because Capcom is a piece of trout and didn't make a billion dollars with MM9 and 10 so clearly those games are not worth pursing any longer. Go big or go home is ruining games in general.