Full-screen attacks

Giga Attack is a full-screen attack that damages or destroys all enemies, usually all seen on-screen. It requires a full gauge, and usually the best way to charge up the energy is by deliberately taking damage. It first appeared in Mega Man X2 as the ability of the Giga Armor's body part, and appeared in all subsequent games after X3; however, some later incarnations do not actually affect the whole screen.

The Giga Attack is not limited to X, however. Zero can also use his own versions of the Giga Attack, as Command Arts learned from defeated Bosses from X4 onwards (the spinoff Mega Man Xtreme 2 also counted). His versions of the attack more closely resembles the original incarnations performed by some of X's armors.

Mega Man Xtreme2

 * X: The armor from Mega Man X2 was also used here, so the same Giga Attack was also available. This is the first Giga Attack to be re-used for a future game.
 * Zero:

Mega Man X4

 * X: The Force Armor and the Ultimate Armor (both introduced in this game) can perform the powerful Nova Strike, a fast flying dash that destroys most normal enemies in its path. Although classified as a Giga Attack, it's the first in the series not to be a full-screen attack. It's also the most recurring Giga Attack in the series, since the Ultimate Armor appears in all subsequent games after X4 (with the exception of X7).
 * Zero:

Mega Man X5

 * X: Unlike the two armors from the previous game, the Falcon Armor's Giga Attack closely resembles the original. The Gaea Armor, however, stands out as having the shortest range for a Giga Attack in the entire series.
 * Zero:

Mega Man X6

 * X: Both Giga Attacks of the Blade and Shadow Armors have limited range, since both involve the use of the Z-saber. The former uses an upgraded Z-saber slash (which can send energy waves forward), while the latter uses the Z-saber to summon two large crescents to spin over and under X.
 * Zero:

Mega Man X7

 * X: The Glide Armor possesses a similar full-screen attack like that of the Giga and Falcon Armors, renamed the Giga Crush.
 * Zero:

Mega Man X8

 * X: For the first time, a Giga Ability is present, instead of a Giga Attack. The ability, called the X-Drive, increases X's capabilities for a short period of time, which is the case for the Hermes Armor. The Icarus Armor, however, has the same traditional Giga Attack. However, unlike previous Giga Attacks, the attack can be used as long as there is WE. However, the lower the WE, the lower the damage the attack inflicts.
 * Zero:

Other Appearances

 * In the non-canon scenario of Mega Man X2 (where you fight Zero before facing Sigma), Zero uses what might be a prototype version of his Earth Gaizer Giga Attack.
 * If you select X in the third fortress stage in Mega Man X5, when fighting Zero he uses versions of his Giga Attack, Messenko and Shin-Messenko. However, these attacks are different from the actual Messenkou attack Zero learns in the same game (the Japanese name for C-Flasher).
 * Copy X's first form uses an angelic version of the Ultimate Armor; he also uses an air dash attack that is slightly similar to the Nova Strike.
 * Omega Zero, the final boss of Mega Man Zero 3, uses weaker, modified variations of Zero's Rakuhouha and Rekkoha. Also, Omega Zero's Kuenzan projects crescents in an arc similar to his version of Rakuhouha.
 * In the commercial for the first Rockman Zero game, Zero was shown creating a powerful explosion after being awakened. The explosion destroyed all enemies, leaving Zero standing strong, which might be a reference to a Giga Attack.
 * Mega Man Model A uses a Giga Crush as its special ability.

Trivia

 * Although Axl doesn't receive a Giga Attack/Ability, some of the attacks he can use while transforming into a Command Mission Boss (e.g. Silver Horn) can hit all enemies.
 * Ironically, the unofficial tribute to Axl, the Model A Biometal from Mega Man ZX Advent, has a Giga Attack, marking the chronological return of the attack to a playable character since Mega Man X8 (although a secret character from Mega Man ZX has the same abilities as the character it's based on).