This is one I've played a lot over the years, between having it on my Wii Virtual Console as a kid and then having easy access to it on my NES Classic and Famicom Mini, but I wanted to review it properly here particularly because I finally beat it on Difficult mode. That was always something I saw as "just too unreasonable" simply out of its reputation, but I finally tackled and conquered it recently, and it really brought a lot of perspective about the game's design to me. I beat the game on Normal and Difficult mode, and it took me a little over an hour to beat it on each.
Mega Man 2's story is pretty bog simple, with it basically just being that Dr. Wily is back and more determined to conquer the world this time! This time instead of just six robot masters, he's brought along eight of them! The stages here, particularly Wily's Fortress stages, are overall longer and more polished than the first game's. Mega Man 2 sets the trend for the rest of the series in taking what came before and making it generally "more" and "better polished". Though not every entry in the series could hold up to that standard, Mega Man 2 is a strong first step forward.
The stage design in Mega Man 2 is very highly improved from the first game. First and most importantly is the fact that Mega Man's movement has been tightened up a LOT. He's still a little slippy, but he controls way more precisely and intuitively than the first game, and it raises the quality of the platforming of the game as a whole significantly. Secondly is that you now get several "Items" for defeating a few bosses instead of just their signature weapons. These Item weapons allow you to skip or make easier certain platforming challenges, although they are required for several sections of Wily's stages. The gimmicks are generally more interesting and fair compared to the first game, and it feels like the game isn't trying to mulch you quite as often. There are still some rough stages, as I think the instant-kill lasers in Quick Man's stage make it a pretty strong contender for one of the worst stages in the NES series, but overall the stages feel fair and fun to go through.
The bosses have been improved for the most part from the first game as well. On Normal mode I find them a little too easy at times, as some die absurdly fast to your normal mega buster weapon, but the last few bosses feel a bit too hard on Difficult mode. I think there's a happy medium possible between the two, but overall each difficulty mode gives you a nice modicum of control over just how hard you want bosses and normal enemies to be (and that's especially true given that this game finally adds E-tanks you can use for mid-fight health refills). Their patterns are more often consistent and readable, with bosses like Bubble Man and Heat Man being super fun fights. However you have some bosses that are just a bit too fast and frustrating to be much fun to kill with anything but their weaknesses, like Crash Man or Quick Man. The final Dr. Wily fights are overall pretty fun, but the fortress bosses are pretty bad. They're either really easy like the Guts Man Tank, or cheap and needlessly difficult feeling like Picopico-kun (the one that is composed from platforms) or the Boobeam Trap (which is easily one of the worst bosses in the entire series). They're a mixed bag of quality, to be sure, but they're ultimately a meaningful step forward for the series.
The presentation of Mega Man 2 is definitely one of the highlights of the series. You have the colorful, quality enemy and boss designs, sure, but the music really stands out here. Damn near every track is one of the best in the series, with the famous first song of Dr. Wily's stages being a well-deserved classic of 8-bit tunes.
Verdict: Highly Recommended. It's not my favorite in the series, and it's honestly not even close, but it's still a game I really like a very good action platformer on the NES. Probably one of the biggest things that would make me recommend it as the first game someone plays in the genre is how easy it is compared to most of the others (although I still wouldn't say it's "easy", just "easier" XD). It's certainly outshined by several of its NES brethren, but I think it's damn impressive just how steep an improvement Mega Man 2 is compared to where the series started.