Game Breaker/Mega Man Battle Network

Strap yourself in, this is going to be loooooong.
 * In general, there are 2 chip that is the single most game changing chip in the entire series starting from the second game, First is Area Grab, which is a simple chip that takes one row from the opponent's field. Second, is Full Cust, which as the name imply instantly fills the custom gauge. The former allows you to control the field, and essentially increases your winning chance by having more space to move, and how easy it is to score your attacks. The latter is a simple, yet deadly tool for increasing your folder speed. It's even more prominent in the second game, where you can have FIVE of them in your folder.
 * Certain attacks are considered game-breaking and thus frowned upon in one-on-one NetBattles. The legendary example is Gater, a Program Advance in Battle Network 2 that deals 900 damage (before boosting), freezes time, and hits everywhere.
 * PA in Battle Network 2 in general, really. Double Heroes hits the whole opponent's field ten times(which screams Attack +). Not to mention that one hit from Bodyguard essentialy locks down enemy's movement for a ridiculously long time, and deals massive damage.
 * And eventually someone decided that PA were too damn good (one can argue rightfully so since almost everyone with a brain had a folder that whored them before the BN5 games) so after the 4th game the PA system was changed to *USE ONE COPY OF A PA PER BATTLE* (instead of being able to form what was first 5 then 4 copies). The effect this had on the competitive scene was a near abandoning of the series after the BN4 games.
 * The first game's Guts Shoot. Its easy to construct since it uses Guard and Dash attack alongside Gutsman, all a pretty common chip in their own right. It does 500 damage to one enemy on the row you are standing on. Consider that bosses max out at 1000 HP...
 * The Toadman navi chip from the second game. Back then, you could have up to 5 of the same Navi chips, as long as you only have, at most 5(or 8 with some style) Navi chips in your folder. The Toadman chip stuns and tracks the target. Basically, you can spam them over and over again, and put in some combo extending chip in the middle of Toadman spam. Oh, and one of the chips from T code(Toadman's code) is Tornado variants.
 * Snakeman from the second game. Easy to set up(some area grabbing, geddon for holes), massive damage especially after boost, and it doesn't give Mercy Invincibility upon hitting. It is, alongside Gater is considered the easiest and cheapest folder to pick up and kill bosses.
 * The Japanese version of EXE2 also had a game-breaking glitch: Throwing a ForestBomb on a Prism would result in neither object disappearing after the hit; instead, the ForestBomb would repeatedly hit the Prism, which would then deflect the hit onto the surrounding 8 panels. This worked especially well on bosses. ** A similar glitch exists with Prism and Varsword. Basically, in 2, Varsword's shockwave is multiplied 5 times when it hits the prism. With some area locking, and timing, you can basically throw the prism, unleash the Varsword shockwave, and win against the boss without much effort.
 * Folder Return is Battle Network 3's resident Bad Idea. Where do I start? It returns every single chip to your folder, including itself. It also functions as a Full Custom, restoring the turn gauge so you can instantly pick your freshly recycled chips and continue your assault. Your opponent also gets the intermission screen, but they're not the ones with a totally fresh deck. And it's got the * code, which means it goes in everything.
 * Don't forget Recycle -- it, too, can, in the right hands, become a Game Breaker. How so? It will literally recycle ANY Navi chip that was used last. That includes fellow Giga Chip Bass (which deals a truckload of damage and normally requires a special panel to exist for it to work), even if the Dark Hole that's needed to use the actual chip is nowhere to be found. Let's also not forget that it can also recycle any of the OTHER Navichips, essentially letting you use the same Navi chip TWICE (which is especially bad for your opponent if you happen to use a multi-hitter that they're weak to -- like BubbleMan or PlantMan). Even better, it keeps any bonuses you had when you used the first chip - e.g. Atk+30s.
 * Don't forget most of the Navi chips. Entire folders have been built around FlashMan, PlantMan and BubbleMan. The first ignores Mercy Invincibility and stuns, the second pierces guards (Looking at you DrillMan), hits multiple times, and immobilizes the enemies, and the last hits an insane number of times (the final one hits around 20 times), with each hit possibly doing over 120 damage depending on how many Aqua +30s you attached.
 * Speaking of BN 3, P code, E code, and F code. Basically, all three of those codes carries several variety of battlechips, including the ability to turn a battle into a complete stunlock. However, the E code stands out the most, thanks to its ridiculous versatility, and near invincible defensive chips.
 * It gets better. In Battle Network 3, thanks to Compression Codes for Navi Customizer programs, it was possible to use the HubBatch program (which gives you All Your Powers Combined, literally - every Style Change from Battle Network 2's signature abilities are given to you and then some, at the cost of a bug that cuts your maximum Hit Points in half while it's set). This includes:
 * Move over any type of panel hazard without ill effect, save one type (poison).
 * Not flinch when hit.
 * Call up a shield that nulls non-piercing damage.
 * Put an extra Mega Chip in your folder.
 * Start the battle with one more Chip in the Custom Menu.
 * Have both your normal and charged attack pierce shields and armor.
 * Be able to survive an otherwise lethal hit and instead be reduced to 1HP.
 * Hub.BAT is a Game Breaker with or without Bug Stopper. You're still at least as ridiculously powerful just by putting it in next to the rest of your favorite programs, even with 500 HP instead of 1000. Of course, it's also an Infinity+1 Sword of the highest order - merely getting it requires you to make it to the last stage of Secret Area (which itself requires several kinds of One Hundred Percent Completion), fight your way through a handicapped Random Encounters run, and then survive through a twenty-round chain battle featuring some of the toughest and most annoying viruses in the game. And then you have to know the key input to shrink it and the enabler code unique to your Style. At least you can still use it on the Bonus Bosses.
 * The enabler code also prevents you from using EX codes, which normally can give you +350 HP for no cost. 350 HP is still nothing compared to the benefits of Hub.BAT / Saito Batch, though.
 * Hub.BAT was nothing compared to 2's Hub Style, which, while it cut your HP in half, gave you the positive attributes of every other style in the game with no elemental weakness included. Yeah. Like its successor, it was also an Infinity+1 Sword that required you to S-rank all the V3 Navis, which in turn required completing the WWW Area, which in turn had other prerequisites.
 * Wood Style, which heals the user while standing on Grass panels, the Undershirt program, which turns a lethal hit while above 1HP into just enough damage to leave you at 1HP, and SetGreen, which made the battle stage start off as Grass panels. Against non-fire enemies and enemies who could not crack floor panels - which included the non-secret final boss - this was unbeatable and simple. (A perfect example of a trick that breaks the game itself into tiny pieces but will utterly destroy you in multiplayer, by the way.)
 * Incidentally, if you get Bug Style (not hard, with the program itself equipped), you can get a part that lets you ignore all bugs, as which the half-HP counts. This dovetails well, shape-wise, with another program that kicks all three buster stats up to the maximum of 5, but also has a bug, this time making it a pain to use chips.
 * Also, Bug Style itself can be a Game Breaker-- occasionally, the negative effects of the style (Forced movement, HP loss, being unable to stand on middle tiles, and your buster glitching up) don't appear and instead you only get the positive effects (Buster Max, a Barrier at the start of battle, or having ten chips selectable at one time). At most, you can get one of the negative effects but all of the bonuses at the same time. Nice.
 * Imagine a magic user being able to cast one specific spell whenever he likes, so long as said spell is on the second page of his spellbook and he's wearing three magic rings on one hand. That's the essence of the 11th Chip Glitch. Initially, players are allowed to select from five chips per round. By earning or trading certain programs, players can boost this up to ten. With duplicates, players can quite literally boost their chip selection up to or past eleven. As the game only has 10 spaces, this results in a glitch which gives players unrestricted access to whatever chip happens to be in the second slot of their unshuffled folder.
 * Dark Chips from Battle Network 4 made the game a joke. If you used them enough when you got the option, you'd eventually get the option of starting out the battle with them. Of course, they removed 1 HP permanently every time you used them, but it was a relatively small price to pay for being able to curbstomp everything in the game in one or two turns flat.
 * Unfortunately, it came with three major drawbacks.
 * First: you can not use Dark Chips against the final boss, so good luck fighting him with reduced HP.
 * Second: It prevented you from using "Light" chips like the NaviSP chips. While this was supposed to balance out the fact that you can now use "Dark" chips like Muramasa, it ended up failing hard, as the Light chips are vastly superior.
 * Third: You'll get raped in multiplayer.
 * Of course, using enough Dark chips means your base max HP will be reduced to 1... But you can easily place a pair of HP+500 programs in your Navi Customizer to counteract this, as customized HP can't be permanently lost from Dark Chip usage.
 * The light path perk in Battle Network 4 is commonly considered to be the most broken passive effect in the entire series. Full Synchro, normally a status effect that is achieved through counter hits, doubles the attack power of your next chip. The light path perk gives you a rather high chance to enter Full Synchro with any successful attack, counter hit or not. This, when used along with chained multi-hit battlechips, can almost immediately destroy any boss that is not immune to being stunned. So broken that the perk was completely removed in the fifth game.
 * The Air Hockey series of battle chips takes advantage of this perk the most. You know it's broken when even the Bonus Boss easily falls to this. That's 3000+ damage in under 11 seconds with no Program Advance in sight.
 * In Battle Network 5, you could combine Dark Chips with Soul Unison to produce "Chaos Unison". It gave you the Dark Chip as a free charge attack. It's at least as buh-roken as it sounds. Toad Chaos and Proto Chaos are popular for getting perfect S-ranks on Random Encounters - a 400 damage BFS or a 300 damage BFG. They only lasted for a turn, but that was generally more than enough.
 * The Chaos Unison had another game-breaking exploit: When fully charged, the charge would rapidly cycle between purple and green. Releasing the charge when it was purple used the Dark Chip with no ill consequences, but releasing the charge on green makes it backfire on you. While they were supposed to have a 50/50 chance of backfiring on you, pausing would stop the cycling, and if it was purple, you could release the B button then resume, and it would successfully fire every time.
 * Shadow Chaos. Its Dark Chip, DarkInvis, makes you invulnerable to ANY attack and causes Mega to go berserk, not responding to your controls... but renders him able to use ANY chip or Program Advance you've used previously.
 * Search Chaos, especially when the enemy is area rowlocked. The target will instantly eat somewhere in the realm of 800-1200 damage as a result. The attack also completely cancels out the two most common forms of defense, Invis and Anti Damage! The only thing that will completely prevent taking damage from this is a Life Aura + Sanctuary combo, and everyone knows how hard that is to set up.
 * Number Chaos. Charge it up and your next attack is now 50 points stronger. A Super Vulcan now does 720 damage.
 * Neo Variable Sword. A mega class chip that dealt 240 damage one square ahead of you, unless you input a special button combo ala most fighting games. Input the right one, and it became 2 hits of 240 to the two rows directly in front of you. Area Steal leaves your opponent with only 2 rows. Proto Soul lets you charge up any sword chip for 2x damage. Combined, you can instantly kill any player in only 2 chips, both undodgable.
 * Suposedly, the guy who created this combo was kicked out of a tournament because he killed his opponent so quickly nobody would believe he hadn't cheated.
 * In Battle Network 3, Variable Sword was an even bigger Game Breaker than its future counterpart. It only did 160 damage, but with the right button combo, this could be turned into four attacks, one in each element, which meant that if your opponent had any sort of elemental affinity, it would do an easy 800 damage (160 x 5) without boosts. And unlike Neo Variable Sword, this version was just a standard chip, meaning that you could have up to four copies of it in your folder.
 * Variable Sword is considered the second most broken standard chip in BN3. The most broken goes to Sensor. What did it do? It put a Killer Eye in front of you that checks in a straight line had it been put on the middle line. If it hits, it does damage that pierces Mercy Invincibility and stuns the victim, and the eye is active for a long time. It doesn't deal as much damage as Variable Sword did, but it forces you to stand on ONE row and being open to attack, or move and open to attack and with proper timing, it can even stunlock.
 * Battle Network 2 is notorious for the Darkness (literally named Dark Messiah in Japan) PA (Bass V3 X + AntiNavi X + Any Gospel chip X), where Gospel appears and uses his dark breath on the front and middle columns of the enemy's side of the field. If anything survived the initial onslaught, Bass would attack the back column. The kicker? Both attacks did 3000 damage each. The first attack alone is enough to kill the final boss himself in one hit-- and it pierces shields, so you don't even have to wait for him to open his mouth!