The following is part of Review A Great Game Day, organized and hosted by 1MoreCastle!
When the question of best Mega Man game comes up, it usually comes down to a barfight between fans of Mega Man 2 and Mega Man 3. Personally I find MM3 a little bit too complicated to be perfect as compared to MM2, but 2's still not the best. I'm not sure if this a controversial statement, or possibly a semi-controversial statement- that's not the goal, but if so, meh -but Mega Man X is the best Mega Man sidescroller.
Set 100 years after the original Mega Man series, it centers around a new kind of robot: X, Dr. Light's last creation. X is the first robot to possess free will (which says something interesting about his predecessor, doesn't it?). Dr. Light, the world's most famous roboticist has somehow died completely alone with nobody to continue his experiments (which says something very sad about X's predecessor...), so X sat in a diagnostic machine for 30 years testing his robot brain for Crazy. Something happened between the beginning of X's generational defrag session and the beginning of the game in 21XX (as opposed to the Mega Man series' 199X-20XX setting) that required X's capsule to be found by Dr. Cain, an archaeologist. You could hang entire games on the between-the-lines stuff in the backstory.
So X wakes up to a world where Dr. Cain has replicated X, and like bad photocopies some of the Reploids are a little... fragmented. So the world is in a pretty serious state of disarray thanks to the Mavericks (that being Reploids whose programming doesn't prevent them from hurting people), and X joins the Maverick Hunters, who I trust don't need much explaining. Their current leader and sole surviving member is Zero, who even more than X is every bit as anime as this plot sounds. Their previous leader Sigma has gone Maverick with most of his troops and is now organizing the renegade robots to war against humanity.
Most of this information is not actually in the game. The plot is left to the manual and other external sources, with the exception of a description of X, which is primarily included to explain what happened to Dr. Light... though not the original Mega Man. I don't think that's ever been explained, which makes sense since his series was and is not over, and who wants to play a game where no matter how many times you save the world you end up dead at the end? (Hi, Mass Effect 3!) The in-game effect is a strange, fill-in-the-blanks bridge between older minimal-explanation games and later, more heavily plotted games. I actually had to look a lot of this up because the intervening years have washed the backstory out of my head. It's too dismissive to say that you should be playing the Legends or Battle Network series if you're playing a Mega Man for the story, but I feel like even with the fragmented in-game plot, it's enough information to intrigue and keep you interested without becoming overbearing as I found the later X games' copious cutscenes. And of course, though I wouldn't have a word for this for another 5 years, Mega Man X feels very anime compared to the original series. More to the point, Mega Man is a 1960s anime/manga (in fact, it's Astro Boy) and Mega Man X is its slick, teched-up `80s/`90s remake.
The game begins with a short prologue level designed to introduce you to one of the new game mechanics, wall-climbing... and otherwise to show you how weak X is in the face of robots designed specifically for combat. The first boss you encounter, Vile (or Purple Boba Fett, if you like) is a fight you're forced to lose so that you can be saved by Zero. Zero is generally set up to be the badass of the game, both because he's the Protoman equivalent (in a lot of ways, as sequels will prove) and because his power is supposed to give you as the player something to aspire to.
The gameplay is the Mega Man format perfected. As always there are 8 Robot Masters (Mavericks this time) with special powers that X has the ability to appropriate, but now there are also a variety of powerups for X himself. While Mega Man's primary sources of power were external devices and boss weapons, it's pretty clear from the outset that X's improvements will be internal. The powerups are presented as upgrades left by Dr. Light in case of emergency. (Let's call them the "In Case I Screw Up" Again series.) The easiest upgrade to get- and the only one it's impossible to miss -is a pair of dash boots which lend the game a speed that the original series never even attempted. Where the original Mega Man games have a largely deliberate pace, MMX speeds things up without sacrificing the precision positioning that makes Mega Man platforming possible. X can dash, cling to walls and dash-jump back off the walls, and there are few to no enemies in the game that can match that mobility once you've mastered it. Other pickups include Heart Tanks that increase X's life bar, and that brings up another interesting aspect of the game. The original Mega Man was always depicted as an equal to his enemies, with an equivalent lifebar. X starts out much more vulnerable, with maybe half as much life- and in Mega Man tradition, most of the bosses have weapons that do more damage to them then they can do to you. Once you've got all the Heart Tanks, you end up with more health than the Mavericks, reinforcing the game's idea that a perfected X will be more powerful than any other robot. Lastly there's the Sub-Tanks, which are a less fiddly version of 199X-model Mega Man's Energy Tanks. Once you get Sub-Tanks they're permanent, and are filled by the extra health powerups the game's drop system generates that, in all previous Mega Man games, were completely useless. In that, they operate like a cross between Mega Man's Energy Tanks and Metroid's.
In fact I'd say that Mega Man X represents a lot of things learned from Metroid and games like it. Stage design allows for a respectable amount of exploration: going back to Flame Mammoth, there's a huge chamber in the middle of the stage that contains a couple of useful items, and is entirely skippable; X can just head all the way to the right and scoot up the wall if you want to avoid the whole issue. Most of the powerups are hidden, some in plain sight, and represent places you'll probably have to revisit later once you've gotten sufficient upgrades for X to be able to reach them. Fortunately the game has reinstated the Mega Man 1 ability to revisit cleared stages- though bosses no longer respawn. The reason for this is another innovation that I don't think was ever used this well in a Mega Man game again: defeating certain bosses changes the condition of other stages in the game. Once you've defeated Chill Penguin, the cooling system in Flame Mammoth's stage malfunctions and the lava solidifies, allowing you an easier path through and access to items previously blocked. While there is, as always, an optimal order for killing the Mavericks, there's not really a required order. The Dash Boots in Chill Penguin's stage make the rest of the game a lot easier, but you can make it through the 7 other stages without them. Later Mega Man games tended to go overboard with the required order in my opinion, making certain bosses practically impossible without the correct weapon.
This game really hammers home the point that the original Mega Man series is anchored to its 1987 origins in a variety of ways, not the least of which is graphics. The X sprite is one of my favorites for its smooth walk animation, to say nothing of recoil when the X-Buster is fired (which X braces with his other hand) and his heavy breathing (he breathes?) when near death- a trait Super Metroid would borrow for Samus the next year. Enemy design is almost as elaborate, and particularly nice is how you don't just get extra damage to hammer home that you've found the best weapon for each Maverick boss, but special animation detailing its effect- Spark Mandrill freezes, Armored Armadillo reels in shock and loses his shields, etc. It was a great looking game in 1993 and still looks good to me today. Still sounds good too- the music joins the gameplay in being faster-paced than 8-bit Mega Man, and the improved SNES sound chip is put to good use, particularly on the prologue stage's theme.
Why is it better than the original Mega Man series? Because it has to be. It's the first chance Keiji Inafune got to have an entirely fresh start on Mega Man, with no reused assets from the old games (unless you want to count Dr. Light or possibly the 1up-bestowing cameo of the Bubble Bats from Mega Man 2.) It renovates things that the original Mega Man series would continue to use for as long as it has lasted, becoming permanent aspects of the series' identity versus its spinoffs and external competitors and making it more definitively a product of its platform and its era than Mario, Sonic, or any other company mascots. In a way, Mega Man X was the beginning of the end for Mega Man, dividing the fanbase and the character with spinoff after spinoff instead of trying to keep the core property fresh and relevant. Because of this Mega Man has continued to assume new identities with new formats and platforms, leaving the older versions adrift as they see no need to adapt to the current gaming reality. Which is kind of sad for a robot whose chief power is to learn from his rivals and add their abilities to his own.
Why is it better than the rest of the X series? For one thing, they belabor the point in ways that put holes in the story: Dr. Light didn't live long enough to let X finish his Craziness Defrag, but enough to make, at minimum, 32 sets of upgrade armor for him? Come on. It causes the series to suffer from the same problem as later Metroid games: somehow Samus has to wind up back at square one and upgrading herself yet again. Worse, later Mega Man X games would betray the same laziness that later 8-bit Mega Man games do, and the attention to small details suffered, as did the difficulty curve. Like Mega Man 2, Mega Man X can be very, very difficult, but once you have the patterns and the powerups it ceases to be a challenge so much as a test of muscle memory. But it's still easier to pick up than later Mega Man X games, which get a little Battletoads. Literally so once the jetbike stages are introduced. And the move to 3D was a halfhearted attempt that once again betrayed Capcom's reluctance to change what works, even once diminishing returns set in... in every sense.
Later chances at real innovation would draw Mega Man out of the sidescrolling platformer arena and create new sub-franchises that would be great in their own right. Later series- Zero and ZX, would go back to sidescrolling with good but flawed results. This is, in my opinion, as good as Mega Man platforming ever got.