The Megaman X Series
Unlike the classic Megaman series, Megaman X takes 100 years after the original Megaman to which either X is really an upgraded Megaman with free will (some references to this happens in X2 and X5) to which most of the robots of the last game have died. So here's the big rundown- Megaman X unlike its predecessor is darker and edgier throughout like the later half of the original Megaman.
We are introduced to the new big bad, Sigma. He is one badass main villain and you'll see why in here...
In here, the gameplay features different challenges like Megaman X has the ability to wall climb but he must also find four different upgrades in order to have a much easier time dealing with the tougher bosses as well as find the heart tanks to increase his life bar. The story starts when former Maverick hunter Sigma goes berserk and some of the best Reploid enforcers have become Mavericks like Storm Eagle and Chill Penguin. Once Megaman gets fully upgraded, he is ready to storm the castle otherwise it takes a super gamer to clear the game without armor which became the standard of sorts.
Megaman X2 continues with the game featuring the mad scientist Serges, Violen and Jygar who are bent on getting the Zero parts. Apparently either Serges is Dr. Wily or a duplicate of Dr. Wily created 100 years ago. The game had the tough challenge of finding Zero's body parts if not Sigma uses them for his own purposes. It also reveals that Sigma can become some kind of computer virus.
In Megaman X3, we are introduced to the short time you could play as Zero but never against the bosses. The story goes with Vile's revival, Dr. Doppler manipulated by Sigma into doing his will and some history on Sigma. For the gameplay, Zero does get cheesy here with his stored shots though this was removed in later games for balancing issues like Bass in Megaman and Bass cannot charge his buster but uses a rapid fire buster. This tag team game was later fully done in Megaman X7-8. Here Sigma creates the Kaiser Sigma body WHICH is the HARDEST Sigma incarnation as in that form, he has no weapon weakness. You'll need the Z-Saber cheat if you want to make things easier but get the bad ending as a result. So Zero managed to "delete" Sigma but he came back under mysterious circumstances. Later games were built during the 32 bit era with the Playstation 1 and the Sega Saturn.
In Megaman X4, Zero is playable for the first time but he has no buster gun, none of those cheesy attributes. Instead, he is introduced as a full-pledged sword fighter. For one, I like using Zero a lot considering that he gets fun when you learn to use his sword techniques. Here you either play as X or Zero for an entire game. For Zero, when he gets a weapon from the boss his sword learns a new ability and they can be done infinitely except for the Rakoha from Storm Owl. This game featured more treachery as Sigma (and Double whose name is meaningful as to mean "Double Agent") manipulate the events. Sigma manages to use Repliforce to further his schemes and destroy humanity. At the same time we learn of Zero's backstory that he is actually Dr. Wily's final robot and implied successor. This game is pretty tragic if you ask me with the deaths of Colonel, General and Iris. X here carries the naiveness of the original Megaman which could be another hint that they are one.
Megaman X5 takes Zero to a more important role. Here when one enters the stage of one's choice, you can select either Zero or X to do the mission. At the same time, you can use Fourth Armor X at the beginning you choose to play as X which shelves ordinary X unless you are using Zero at the beginning (who has a mele buster shot). In this game, Sigma tries to collide the colony to Earth awaken Zero from his "false self" which proves he is plain sick. Here there are two different armors available namely the Falcon Armor and the Gaia Armor which cannot be used until all four parts of each are complete. However one cannot swap into different modes like in Megaman 6, you played the whole stage with that armor and the Gaia Armor like the Power Megaman had limited range of shots though it can walk on spikes. This was supposed to end the X series but demand had it for another game.
Megaman X6... probably the hardest game of all. You had to rescue Reploid hostages and if not, the games main threat the Nightmare Virus will take over them and you have to kill them. Hard game but pretty fun as the controls aren't a nightmare so you have the right balance. You had to fight Zero as a secret boss to play as him and Zero has a more reliable Z-Buster shot but still can only be fired when he's standing and of course, he's still a sword player at best. Here we are introduced to Isoc who might be either Serges rebuilt or Dr. Wily himself, which is unknown as Dr. Wily knows much about Zero. So there's no official explanation but Dr. Wily's memories could be in Serges. The main antagonist Gate is studying the Nightmare Phenomenon which soon led to Sigma's revival.
In Megaman X7, we are introduced to Axl and tag team gameplay. One chooses two characters who can tag in and out. But the game starts with Axl who is a cocky mercenary and sort of rivals with Megaman which either he is based on Bass or he could be Bass reconstructed since he almost has an identical gameplay to Bass in the "Megaman and Bass" game. What makes Axl cool is that he can also mimic certain enemies for a short time if they are finished with his Copy Shot. In here, Colonel Red was manipulated by Sigma into another rebellion. It's quite understandable how X gets to be overly whiny here considering the fact that Sigma is really the most cruel Megaman villain ever. But WTF why do I have to beat all eight Mavericks before I can play as X or the alternative, rescue 64 reploids? It gets really annoying.
In Megaman X8, this seems to wrap up the X series and seems to end Sigma. So here we have the plot of the Jakob Project to create a new utopia in space. As the conspiracies are solved by X, Zero and Axl one can see that Sigma gets his comeuppance. For gameplay, this game involves Megaman being able to swap parts for both armors and also, unlocking various secrets which can be carried over in the Replay Mode as well as the fun of having the hugest combo ever. Lumine was working with Sigma all along, however Sigma got his karmic end when Lumine manipulated him behind the scenes (which his insanity didn't figure things out). So while Lumine is actually working for Sigma, Lumine turns his back on Sigma but is immediately destroyed with no chance of coming back. Sigma hasn't returned ever since.
In Megaman X Command Mission, it's an RPG instead. For me, this game featuring the story of the rebellion and a secret conspiracy from within was really a great game to play. Unfortunately the Command Mission came didn't spawn new games.