Mega Man 2

Mega Man 2, known as Rockman 2: Dr. Wily no Nazo (ロックマン 2: Dr.ワイリーの謎) in Japan, is the second game of the Mega Man series and is considered by many fans the best Mega Man game for the NES.

Overview
Dr. Wily is back, and now he's created eight new Robot Masters to defeat Mega Man. Mega Man will have to stop Dr. Wily's robots and bring him to justice again.

There is no difficult select in the Japanese version of the game. There is a normal mode and a difficult mode in the English versions, which Difficult is the original Japanese mode and Normal is an easier mode in which Mega Man's attacks causes twice damage. The difficult select is only available in the American NES and mobile phone ports of the game.

Story
From the game:

"In the year 200X, a super robot named Mega Man was created. Dr. Light created Mega Man to stop the evil desires of Dr. Wily. However, after his defeat, Dr. Wily created eight of his own robots to counter Mega Man."

Trivia

 * This is Keiji Inafune's favorite Mega Man game.
 * This is the first Mega Man game that shows the map of Wily Castle.
 * This is the first Mega Man game to have Dr. Wily create his own set of Robot Masters, instead of using someone else's.
 * This is the first Mega Man game to have the "Teleport System" when encountering the Robot Masters again.
 * This is the first Mega Man game to have Energy Tanks, although the player could only carry four, instead of nine as in later installments.
 * Mega Man 2 was made into an incredibly stripped-down handheld electronic game by Tiger Electronics.
 * Every level is, in essence, the same; Mega Man must run to the right a specific distance while attacking or avoiding two differing enemies and leaping on two occasionally disappearing platforms until he reaches the area's boss. However, the player can still select any Robot Master level in any order.
 * This version had only six of the original eight bosses (Wood Man and Crash Man are not present) and the graphics art for Mega Man borrows from the cover art as well as the original graphic (he attacks with a pistol instead of an arm cannon which, strangely, has a limited amount of weapons energy). The bosses that exist also bear little resemblance to their NES counterparts.
 * Of the six bosses to fight, only Quick Man, Heat Man, Bubble Man and Metal Man have their regular sub-weapons in check; Air Man and Flash Man fire out bullets similar to Mega Man's gun that can actually counter each other out. As a result, their sub-weapons are another batch of regular bullets.
 * The bosses do not possess any energy bars and can be destroyed by either fifteen shots with the regular shots or three hits with their specific weakness.
 * Once all six are destroyed, Mega Man then chases after Dr. Wily in his escape pod until he reaches the end of the level. However, like the original version, Bubble Lead is still his weakness, as no other weapon will damage him in the ensuing fight.
 * Like the first game, using a Game Genie on Mega Man 2 will cause the music to be riddled with quirks and odd sounds.
 * Quick Man's stage is taken from this game and is used as Dr. Wily's fortress stage in the Game Gear Mega Man game.
 * Also, Shadow Devil's stage in Mega Man X5 and QuickMan.EXE's stage in MegaMan Network Transmission are similar to Quick Man's stage.
 * Despite the original Mega Man having an alternate music track for fortress bosses, Mega Man 2 lacks this feature.
 * Many ideas that were not used in the original Mega Man game, due to cart size limitations, were recycled for Mega Man 2.
 * This is the only Mega Man game so far to have a Robot Master that is weak to his own weapon.
 * Seeing how Metal Man is Inafune's least favorite Robot Master from this game and Wood Man is his favorite, it is possible that their serial numbers are ordered from Inafune's least favorite to his most favorite (Metal Man is DWN. 009, whereas Wood Man is DWN. 016).
 * The same thing could be said about the first game.