Game Breaker/Mega Man

Game Breakers in the Mega Man series.

The "Rockman Complete Works", PS1 ports of the NES games, provided a feature using the Sony Pocketstation, a Japan only portable device, that featured mini games that allows the player to level up Rockman, and then add that data back into the main game. Stats boosts include more firepower, more health, and greater speed. While this feature was removed when the games were put into the Anniversary Collection this is a sample of what could've been.
 * The aforementioned Anniversary Collection featured an easy mode and the option to start with four extra lives. The extra lives is self-explanatory, but the easy mode provides better defense, stronger weapons, and less enemies in the NES games. 7 has all the weapons at double power compared to normal settings. Sadly this doesn't affect MM8 or the two arcade games.

Mega Man 1

 * The Thunder Beam (called "E" in game). Shoots straight, up and down at the same time, the damage amounts are godly and the ammo consumption is ONE bar per shot. Tied in God-tier with Metal Blades. This game also has Fire Storm, called F. It raises a shield that, by itself, can almost kill a Joe in one hit. Plus to that, a large fireball that moves quick and deals great damage. Consumption? 1 bar per shot. YES.
 * And, add to that the "Select" trick, which basically allows you to one shot the Yellow Devil.

Mega Man 2

 * Metal Blades, the weapon you receive after defeating Metal Man (who is arguably one of the easier bosses to deal with when just using your default Mega-Buster weapon). Once you realize they can be fired in any direction (something the Mega-Buster can never do in any of the games), they often mow straight through swarms of smaller but still troublesome enemies and that you almost always have an abundance of weapon energy for it, the idea of using the default weapon at all pretty much goes straight out the window.
 * To put the abundance in perspective, you have four Metal Blades for each bar in the Energy Meter. There are twenty-eight bars in that meter; do the math.
 * That's 112 blades of doom. 28 * 4 is 112. My math is correct.
 * The ammo amount doesn't even matter if you abuse a glitch in the game that gives the Metal Blades (and any other weapons that get more then one shot for each bar) infinite ammo.
 * And when you face Metal Man again during Wily stage 5, he goes down from two hits of it (or one on the US version's Normal mode).
 * With it you can defeat not only Wood Man but Bubble Man and Flash Man as well. That's half of the Robot Masters.

Mega Man 3

 * The Rush Jet, which, unlike other games, lets it move freely in every direction. You can pretty much negate any set of Bottomless Pits or Disappearing Blocks by using it. The worst thing about it is that energy is only drained if you're standing on it, and it follows you left and right even if you're jumping or on another platform entirely. This means you can cross large gaps with virtually no weapon energy loss.
 * 3 had a lot of tricks that could count here, though most were removed for the anniversary collection. Just by going to the password screen and putting in A6, either color, you start a new game with nine E-Tanks (that stayed in the newer versions). There are glitches for getting Rush Jet and Marine early. That and the controller 2 tricks.

Mega Man 4

 * The Pharaoh Buster, which could be charged up and thrown in any direction much like the Metal Blades from the second game. It was extremely powerful and charged up quickly, but the best part was that it completely destroyed most enemies and bosses with ease, and this included the final boss, Dr. Wily.

Mega Man 5

 * The debut of Beat. Granted getting him required finding all eight letters in the eight robot master stages (and thus only usable for the second half of the game on), Beat was notorious in the fifth installment of the series for being Mega Man's version of Cream the Rabbit's Chao companion, Cheese. As long as you have weapon energy, Beat will chase down any enemy onscreen when summoned and instagib it. It doesn't stop there, Beat also works on bosses; he even deals as much damage to them as a weapon a robot master has a weakness to. In fact, Beat was so broken he even completely trivialized an otherwise very annoying encounter with . It was so bad that Capcom changed his gameplay function to only rescuing Mega Man when he fell into Bottomless Pits from 7 on. In 6, he was still game breaker, but he no longer hunted down Bosses for you.

Mega Man 6

 * The Jet Adapter (Rush Jet Adapter in some regions). Although you were only able to fly for a short amount of time, and could not slide, use special weapons, or charge your buster while using it, this armor made the platform-heavy sections (and several other areas) a joke, as it's easy to use and it recharges very quickly when you are on the ground or on a ladder. Given the nature of the games, there's little reason to use the other form (or arguably normal Mega Man) over it except for cases where sliding or a special weapon is absolutely necessary. Getting this item is one of the first things many players do (as the robot master who has it is one of the easiest of the set) when starting a new game.
 * The Power Adapter wasn't too bad either. The charge shot, despite having almost no range, could wipe out shielded enemies, like a guarding Metool for example.

Mega Man 8

 * The Arrow Shot upgrade was a bit of a gamebreaker too. This thing could kill nearly any stage enemy in one hit, shielded or not. And the plasma spread after it hit something could hit something behind your first target. The Laser Shot could hit anything as well, but seemed to have less stopping power.
 * The Laser Shot also charged ridiculously faster than most of the other weapons, so it was definitely a Game Breaker.

Mega Man 9

 * This installment gives us an entire slew of Game Breaker weapons: Jewel Man's Jewel Satellite is considered the best barrier weapon in the series, combining the best qualities of the Leaf Shield, Junk Shield and Star Crash with the ability to reflect your opponent's bullets back at them for damage; Splash Woman's Laser Trident, which is the armor-piercing variation of Metal Blade on steroids (unidirectional, though); Hornet Man's Hornet Chaser, a more versatile (and homing!) Grab Buster; and Galaxy Man's Black Hole Bomb, a controllable quantum singularity that eats virtually any enemy in the game in one sitting. The lack of the Charged Shot and slide in the game is more than made up with by these babies.
 * The Black Hole Bomb eats a lot of weapon energy per shot, and you can only deploy one at any given time (though the pause trick does circumvent this to allow for rapid redeployment). The same applies to the Jewel Satellite, but you only need to have one for that. The Laser Trident only fires straight as a counteract to the Metal Blade philosophy, and the Hornet Chaser doesn't have too much in terms of power, especially against non-female bosses. That balances them out better, but not by much.
 * Also, the Jewel Satellite gets much more versatile once you get the Energy Balancer. Once you've recharged the energy you used to fire JS, any extra weapon energy you get will go straight to the weapon with the lowest energy level thanks to the Balancer.
 * It gets even worse when you have both the Jewel Satellite and the Energy Balancer. With this combination, you can go to Plug Man's level and literally activate the satellite under the first hole in the wall which spawns infinite minions that attack you from above and drop their items on you until you decide to throw the satellite away. This means you can essentially put Mega Man in the right place, put the controller down, leave for a few hours or so, and come back to find that you probably have enough bolts to make anything else you will need in the game. This is also the best way to get the trophy for getting the maximum amount of bolts at one time. The best part about this strategy: The Jewel Satellite is super effective against Plug Man.
 * Not even the Mets are safe from your wrath thanks to the Black Hole Bomb and Tornado Man's Tornado Blow. You know it's broken when it can destroy a Mettaur even if it is hiding in its shell.

Mega Man 10

 * Bass. He still has many of his powers from & Bass except the double jump, he can't shoot through walls, and his shots are half as powerful as Mega Man's, but he can still fire in seven directions and has a rapid auto fire to boot, making a lot of bosses and damn near everything else cannon fodder. He also seems to take half the damage of Mega Man as well.
 * This advantage is somewhat negated by it being possible for Mega Man being able to have the Ballade Cracker by default, which is a gamebreaker in its own right. It's got Metal Blades' 8-direction brokenness, but in exchange for not going through things or ridiculously low energy usage, it has an explosive blast radius around it. In it's original appearance, this was balanced by recieving it near the end of the game, but beat Ballade once in 10 (via Special Stage 3), and you permanently have it from the get go.
 * Speaking of bladed weapons, the Triple Blade received by Blade Man definitely qualifies. Granted, Blade Man was one of the tougher, if not the toughest of the Robot Masters in that game, but if you got his weapon, you could defeat most of the other bosses in the game pretty easily, especially  who otherwise is a complete pain to deal with.