I find it interesting that you folks say Mega Man 2 is the best game in the series. I'm actually starting to wonder if the game is overrated. The stage design has potential, but there are an awful lot of respawning enemies which bring it down. The weapons are pretty imbalanced, most of them are pointless due to the awesomity of the Metal Blade. The robot weaknesses are kind of messed up, a lot of them take damage from three or four different weapons. Plus, four of the robots are also weak to the Mega Buster, instead of just one or two like the other games. To be fair, when creating this game, Capcom was coming off the first Mega Man with which they received complaints that it was too difficult. At this point they were still trying to find a good balance of difficulty.
The reason, I think, that Mega Man 2 is so highly regarded, is because it was the first or second Mega Man game that a lot of us played. It was also quite an improvement over the first game, and offered more new features than any game after it (energy tanks, 8 bosses, passwords, help items, Dr. Wily castle intro, etc.). The main thing which makes it memorable would have to be the music. This game was released at a time when music in videogames was treated as an afterthought, its existence was merely to create sound just so games weren't silent. The Mega Man games actually put focus on not only composing good music, but also creating a different music piece for each stage. Ask anyone about Mega Man 2 and I guarantee you the first thing they will mention is the music; it seems it is what leaves an impression on people above all other aspects.
For the record, Mega Man 2 is my favourite game in the series. From an objective standpoint however, I don't think it is the best game in the series, and certainly couldn't be argued as fact.
I'd like to offer a slight correction. You guys mention that previous Mega Man games had passwords for each stage of Dr. Wily's castle, but that isn't true. However, apparently, Mega Man 4, 5, and 6 all had two castles, and for these, there is a password for Dr. Wily's castle so you don't have to replay the first castle. There still isn't a password for each stage, though.
Also, the M-Tank is a Mystery Tank, not a Mega Tank. In Mega Man 5 when you use the Mystery Tank, it would turn all on-screen enemies into a 1-Up. I have not tested it out in MM10 to see if this is still the case.