Minor Spoiler Warning. Some story elements are revealed.
The Mega Man franchise has pretty much hit a brick wall after the release of the Mega Man 10. Capcom has pretty much regulated Mega Man as a guest characters for other games and saw fit to cancel a sequel to Legends series. Keiji Inafune, Mega Man's creator, has left Capcom and is set to release his new Mega Man-esque game, Mighty No. 9. These are dark times for the blue bomber whom Capcom has pretty much shelved at this point. Mega Man has become nothing more than a nostalgia piece that gets pulled out once in awhile to make the fans happy.
Looking back on the classic series, many players have a strong fondness for the older games and most gravitate towards Mega Man 2. Mega Man 2 is a great game with lots of memorable moments. Who doesn't love using the overpowered metal blades from Metal Man? Or how about the first time you encountered the huge Mech Dragon and Guts Dozer? Or the epic intro leading up to the universally loved title screen music theme?
Mega Man 2 in all aspects was bigger and better than its predecessor. It is one of the best sequels to come out for any franchise in the 8bit and 16bit era. However as the Mega Man franchise continued it hit it's peak pretty quickly with what I consider the best game in the series, Mega Man 4. Mega Man 3 was a very good entry in the series but wasn't as great of a game as it could have been.
Mega Man 3 had the introduction of your robo pet companion Rush, a new slide move, 8 new interesting robot masters, great music and the nostalgia trip with the Mega Man 2 robot masters. Despite all these awesome additions the game was plagued with massive slowdown in a few stages, uninspired level design, a useless Rush Marine form and rather disappointing final boss.
Mega Man 4 improved the series in a lot of ways despite having a couple of minor flaws itself. The big addition to the series that Mega Man 4 added was a narrative. Mega Man 2 and 3 had endings where they touched on the story provided in the game's manual booklet. Mega Man 4 was the first in the series to have a opening cutscene that delves into the series history briefly then proceeding to the present day dilemma.
For the first and only time, a new villain appeared to challenge Mega Man. We were introduced to Dr. Cossack, whom we later learn in the game has a motive for why he attacked. We learn Cossack was forced to do Dr. Wily's bidding because Wily held Cossack's daughter prisoner. Yes, it turns out that Wily is indeed the true villain of the game, but it was a nice plot twist showing off how dastardly Wily could be. Having two villains in the game also allowed for it to be longer as well. You have to battled your way through Cossack's castle and now Wily's castle to end the game.
Sadly this villain fake out would become a trend for the last two NES Mega Man games and with less impact to the point that it almost destroys the effort of the plot twist in Mega Man 4. Mega Man 5 had a fake ProtoMan that was controlled by Dr. Wily and in Mega Man 6, Mr. X was Dr. Wily in disguise. Those were horrible plot devices for the sake of just making the games longer.
The music for whatever reasons seems to be brushed aside when people are asked which Mega Man game has the best soundtrack. Many of the robot master stages in Mega Man 4 has great tunes and possibly the best ending theme in the series. Stage music like Skull Man's theme have a great fast paced beat that it keeps you pumped up. Many of the stage tracks in my opinion are equal to many of the great tracks from Mega Man 2 and 3. Mega Man 4 is also the first in the series to have a new final boss battle theme that was unique. The Wily final battle tune is incredibly intense sounding and one of my favorite tunes.
Gameplay wise, Mega Man 4 was pinnacle of the series. It was well balanced in terms of challenge and level design. All the levels were designed to put all that you learned from the previous Mega Man games to the test. Some levels like Pharaoh Man's level had some moving platforms you had to jump to all the while not falling to a spiky death and avoiding the flying heads from robotic mummies. Frog Man's rainy segment was very misleading as you actually have to deal with the wind when making jumps. One jump in particular forced you to jump a wide gap into the wind and rain making it a very tricky jump.
The Robot Masters were no longer a true walk in the park like in previous games. You no longer had the advantage of being able to use more than 1 weapon to kill them quickly. Damage done to robot masters were reduced significantly. No longer were they dying in 3 to 5 hits from an over powered weapon. The damage received from a robot master was also increased as well. Also while they still retained patterns just like every robot master before, a couple of them were able to change their pattern once in a while, like Drill Man, Skull Man and Dust Man.
All the Robot masters in previous games didn't offer that much of a challenge aside from a few. Those few that did usually had abilities like Air Man's attack or a room that required lots of platform jumping like Flash Man or Snake Man. Like I mentioned, usually most of the early robot masters could be defeated quickly with a number of different weapons. In Mega Man 2, a lot of the masters could meet their demise with the Metal Blades. In Mega Man 3, Hard Man and Needle Man's weapons worked on at least two masters each and almost all the Wily Castle bosses.
Speaking of the Wily Castle bosses, in Mega Man 4, they all have basic patterns but can deliver some pain. The giant Hard Hat Mac in the first Wily stage was an intense battle as you had to time not only when to slide under his jumps but had to jump into the air to avoid being stunned when Mac stomps the ground. However this may be one of the two things about Mega Man 4 that I take issue with. Battles like The Mecha Dragon forced you to use those 3 blocks to dodge incoming attacks or risk falling to your death, or the Yellow Devil made you learn his pattern when moving from side to side or else get hit with tons of potential damage before ever laying a hit on him. In Mega Man 4, you fought the giant bosses on an even playing field.
Aside from the new Robot Mater weapons, we finally get a new charge attack, the Mega Buster, for Mega Man. This quick charging move allows us to fire a huge cannon burst for extra damage to all enemies. This much loved ability will become a staple for all future Mega Man and related games.
It's also good to point out that the weapons you get all have good uses for speed runners. Granted in our childhood when this game came out, speed running wasn't even really a thing. To this day, Mega Man 4 seems to be the go to game for people who love to challenge themselves besides Mega Man 2 and 3. In fact, during the 2015 Summer Games Done Quick Charity event, there was a Mega Man 4 race between two speed runners and it was one of the highlights during the event.
Even though I don't partake in speed runs, I still find myself playing through Mega Man 4 over any others in the series. Nostalgia glasses put aside it is a well built game from beginning to end. I loved the graphical design of this game, it's the most balanced and challenging in the entire series, it has several memorable tunes, and it brought story telling and narrative to the franchise. There are few if any faults with this game(rush marine is still useless) and it made me a fan of the Mega Man franchise from then on when it came out.