Mega Man X6

Mega Man X6, known as Rockman X6 in Japan, is the sixth game in the Mega Man X series and was the final title in the series to be released on the PlayStation. Both Mega Man X5 and Mega Man X6 were released after the launch of the PlayStation 2, so after the development and release of Mega Man X6, Capcom made the transition to the PlayStation 2. Mega Man X6 was developed and released on the original PlayStation mainly to reach out to a broader audience. The game was later re-released as part of the Mega Man X Collection.

Series creator Keiji Inafune originally intended for the series to end with Mega Man X5. As such, it caused a change of plans in the Mega Man X series, since it was originally intended to have Zero resurrected in the Mega Man Zero series. Thus, Mega Man X6 is the first Mega Man game not to be directed by Inafune. 

This game is by one of the most challenging and hardest game in the whole X Series.

Story
Three weeks after the crash of the Eurasia colony in Mega Man X5, the Earth's surface is badly damaged, and humans have been forced to live underground to survive from the pollution. Reploids have been sent to the surface to attempt to make it liveable once again. There was no trace of Mavericks for some days, and the Maverick Hunters help cleaning up the world. Gate, a Reploid scientist who was once a former colleague of Alia, stumbles upon the crash site of Eurasia, finding something unusual. After that, Gate became mad and a strange set of Maverick outbreaks occurs, and when X is called to stop a giant Mechaniloid, he sees a purple-like figure that resembles Zero. Around the same time, a powerful Reploid named High Max appears and claims to be investigating the Zero Nightmare, and they fight. High Max, however, proves to be too strong and X cannot damage him with his X-Buster.

A Reploid scientist named Isoc makes a announcement saying that the ghost of Zero, Zero Nightmare, is behind the new Nightmare Phenomena. He sends eight Reploids to investigate areas with high jamming suspected to be infected by the Nightmare to discover the mystery surrounding it and delete Zero's ghost, with High Max leading the mission. Isoc also ask for volunteers to help them. X goes off to the suspected areas to investigate the Nightmare, defeat the Mavericks, and rescue the Reploids that volunteered to help.

During his mission, X fights against the Zero Nightmare and finds the real Zero, who was believed to have been destroyed after the last encounter with Sigma. Signas is reluctant to ask Zero's help as he had just return, but due to the crisis, Zero helps X in the mission.

After defeating the Mavericks, the Maverick Hunters discover Gate was behind the Nightmare Virus. Gate is impressed with the abilities of X and Zero, and challenges the Hunters to a duel, revealing them the location of his hidden laboratory. In his laboratory, Gate also reveals he was able to create the Nightmare and High Max by analyzing Zero's DNA, which he found in Eurasia's crash site. X and Zero defeat High Max and Gate; Isoc was later found unconscious in a similar way to the Erasure Incident (mentioned by Alia when playing as X). Upon Gate's defeat, Gate reveals he had revived Sigma. Sigma states that he didn't need his help to revive, attacked Gate and retreated. X and Zero rush to stop the partially-reconstructed Sigma and battle him, and although he was not fully complete, he puts up a good fight. In the end, the Hunters prove victorious, defeating Sigma and removing Gate's body from the wreckage of his lab to attempt to repair him.

Bosses
Eight Mavericks:
 * Commander Yammark
 * Rainy Turtloid
 * Shield Sheldon
 * Blizzard Wolfang
 * Blaze Heatnix
 * Infinity Mijinion
 * Metal Shark Player
 * Ground Scaravich

Others:
 * D-1000
 * Zero Nightmare
 * Dynamo
 * Nightmare Mother
 * High Max
 * Gate
 * Sigma

Armor
X's Armors:
 * Falcon Armor - X begins the game with this armor.
 * Blade Armor - Manuverability and Z-Saber-based armor.
 * Shadow Armor - Defense and stealth-based armor.
 * Ultimate Armor - Accessible through a code.

Zero's Armors:
 * Red Armor - Zero begins the game with this armor.
 * Zero Armor - Accessible through a code.

Trivia

 * In gameplay, Mega Man X6 is the only game where it is impossible to acquire both the Ultimate Armor and Zero's black armor in one playthrough (or save file) without using external cheats. X4 is also in a similar situation, although it's only because X and Zero are playable separately.
 * This is one of the few games that doesn't feature Sigma as the main villain, a first in the X series. That honor goes to Gate, though Sigma was still included as the final boss.
 * This is the first game in which the Reploids in the game could be corrupted, collected, or be missing. It was also the first to mention they could give X and Zero upgrades.
 * Zero's ending in the game is supposedly the true ending of the Mega Man X series (however, the ending was originally planned to be one game earlier). It was never officially stated when this ending takes place, allowing three sequels (Mega Man X7, Mega Man X8 and Mega Man X: Command Mission) to be made so far.
 * This is the first game in the X series where most of the eight Maverick's names were not differentiated from the Japanese release for the US release.
 * The Reploids that are able to be rescued in Blizzard Wolfang's stage share the same names of characters from other Capcom franchises such as Street Fighter, Resident Evil, and Devil May Cry. (Ryu, Ken, Leon, Dante, etc.)
 * This and X3 are the only X series games in which the heroes (X and Zero) can use both a Buster and Saber.
 * Oddly, the intro says that the events of Mega Man X6 took place three weeks after the events of Mega Man X5, whereas in X5, the ending stated that it took place three years later. Though it's simple to assume that the events of Mega Man X5 took place in a possible future time.