In general, there are 2 chips that are the single most game changing chips 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.
The first game has pretty much any chip that deals ≥100 damage, a much lesser example. The reason is that enemies rarely had health over 250 HP, and bosses capped out at 1000. Not to mention, all the upgrades for your standard chips like Cannon and Shockwave would have 120 and 100 damage respectively - and Dynawave was quite fast. In later games, the focus shifted more to multi-hit attacks.
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 Custom 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 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. 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 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 Navi chips, 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 10 times), with each hit possibly doing over 120 damage depending on how many Aqua +30s you attached.
Speaking of BN3, P code, E code, and F code. Basically, all three of those codes carry several variety of Battle Chips, 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.
not just the stunning chips these 3 codes also have access to several extremely reliable and powerful Program advances and chips to make use of the plentiful stunning chips the E code alone has the most of any single letter coded P As and the F code gets the Hyper Ratton PA which meshes extremely well with Flashman powerful stunning capability and P code gets access to use evil cut PA which chews up most things one-on-one as well has like hyper ratton benefiting from a navi that has a stun effect (plant man) to ensure you can land it.
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 - 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.
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.
Anti-damage. There are several chips of this type, but each are specific counters to types of chips (Anti-fire, anti-wood, even Anti-navi and Anti-recover), but as the name implies, Anti-damage will counter ANY source of damage. By which we mean it negates the attack and fires back with a near instantaneous barrage of nigh-undodgeable shurikens. BN3's Anti-damage fired off three shurikens for 100 damage each, and these chips were UNLIMITED. It was nerfed in later games to only throw one, classified as at least a mega chip and made so some attacks could get through, but still a respectable chip in it's own right.
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 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 (it stops at 499 uses, so you'll end with 501 HP if you collected every HP Memory) and caused glitches during battle, but it was a relatively small price to pay for being able to curbstomp everything in the game in one or two turns flat. Also, once per battle, being deleted while having low karma triggers DarkInvis instead, causing MegaMan to become completely invincible for a while and attack randomly with any attack used through the game, including Program Advances.
Unfortunately, it comes 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 prevents you from using Full Synchro, Double Soul and "Light" chips like GunDelSol, Sanctuary or Navi chips other than those from villains. While this is supposed to balance out the fact that you can now use "Evil" chips like Muramasa, Anubis and DS Navi chips it ends up failing hard, as the Light chips are vastly superior.
Third: You'll get wrecked in multiplayer.
Of course, to compensate for the permanent reduction of HP you can easily place a pair of HP+500 programs in your Navi Customizer, 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 battle chips, 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.
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. Proto Chaos and Toad Chaos are popular for getting perfect S-ranks on Random Encounters - a 400 damage BFS or a 300 damage BFG that wipes the entire field. They only lasted for a turn, but that's if somehow you need to enter the Custom Screen again. The way it works is that the charge cycles between purple and green (the timing changes with each successful use). Releasing the charge when it is purple uses the Dark Chip with no ill consequences, but missing the timing and releasing the charge when it is green again disables the ability and releases an invincible DarkMega on you. While Chaos Unison was supposed to have a 50/50 chance of backfiring, the timing for attacking isn't too hard to get right at least twice and you can also abuse the pause button to stop the cycling. If it is purple, you can safely release the B button and resume, for it will successfully fire every time.
Shadow Chaos. Its Dark Chip, DarkInvis, makes you invulnerable to ANY attack for a long period and causes Mega to go berserk. Althrough you can order him to use other equipped battlechips, he teleports around and doesn't respond to other controls... while able to use ANY chip or Program Advance you've used previously. It's a random ability so sometimes Mega Man might waste it on normal Buster shots, but there no downside whatsoever to playing with it.
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. This makes some multi-hit attacks extremely powerful without the aid of other chips; for instance, the Super Vulcan mega chip now deals 620 damage.
Also in regards to Battle Network 5, the S code is notorious for being rich in practical chips with respectable power. With the Life Sword, Big Noise and Wild Bird Program Advances available, it allows one to at least have a easy time playing the game to the end.
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 (DarkMessiah 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!
Arealocking your opponents, then using prism with Bass/Serenade allowed you to deal massive damage to your opponents. However, 4-6 made it so your opponent's back row could not be captured, but it still applied with Bass if you used it with various Att+/Color Point chips, and in Full Synchro/Angry state. The same chips and states could be used with Super Vulcan instead, creating the Silver Bullet combo.
However one of the most infamous, and due to it's nature farthest reaching combo, the Silver Bullet Combo. Essentially, when an Atk+10 chip or something like it is used on a multi hit chip, that modifier applies to each hit. Same goes for Full Synchro. The Silver Bullet involves using attack boosting chips (And optionally a stunning chip) with one of the Vulcan Chips. Now, with your standard Vulcan chip (3 10 damage shots) this isn't so bad... but in later games, they introduced a Megachip known as the Super Vulcan (12 shots) and they had the Color Point chip mentioned above. And every shot doesn't allow for mercy invincibility, and stuns the victim long enough for the next shot to hit. With Color Point, that's approximately 70 damage per shot, which is 840 damage with two chips.
For best results, use MetalSoul, which can get double damage (and Break effect) just by charging up — no need to count on Full Sync kicking in. Is it any wonder Break-type souls were nerfed after this game to stop affecting neutral chips?
Or in BN5, Chaos Number Soul. Its charge attack is Dark Plus, which means +50 damage for the selected chip.
Ever wonder why area locking was nerfed after 3? There were quite a few chips that deal more damage depending on how little space the opponent has. Like Meteor, which deals 40 per hit, and normally has 27 hits spaced out over 9 tiles, meaning a single target (which takes up only one) will get hit 3 times, for 120 damage. Not so bad, right? Arealock the target to one square, and you have a single target being hit 27 times. That's 1080 damage. Oh, and it's multi-hit, which means you can add atk+10. With 4? That bumps it up to 80 per hit, which doubles the output to 2160 damage. But it's also elemental, specifically, fire elemental, meaning you can add the element damage boosting chips, specifically fire+30. With 1 of those, you get 1890 damage, with 4 you get 160 damage per hit, which does 4320 damage total, 4480 if Grass Stage is set up, and 8960 if the opponent is weak to fire.
Another combo that involves area-locking is Snake. For every hole on your side of the field, the opponent is dealt 40 damage. Normally you either trap yourself with holes setting this up, or you area lock your opponent and make holes everywhere possible. One of the extra-folders even has this combo built in, which shows the developers were aware of this, and is one of the recommended ones if you want to complete Serenade's Time Trials. Putting it in your own folder makes it even more powerful. Since it requires at least one space on your side to not be a hole, and it only has one snake per hole, it does 16 hits so it's not as much as Meteor... except that where Meteor is a Mega Chip, Snake is a regular chip meaning you can have multiple snake chips. Utilizing Area Lock and Geddon 2, one can easily set the stage up, and then flashman to cause a stun at the end of a round before going to the custom screen, you're free to fill your folder with snake chips and Wood+30s. The best part is, even with all the damage done, the opponent acts as if they were hit with a very weak attack, only having a slight flinch instead of mercy invincibility. They can be hit during this flinch. What do you hit them with? The other snakes you chose at the beginning of the round. You can combo at least two of the chips, and with proper timing or Button Mashing up to 4. If you can only get 2, you can simply choose two snake chips and load one with 3 Wood+30s as it is an elemental chip, and get up to 2720 damage, 5440 if the opponent is weak to Wood. If you can combo 3? 1920 at base, 2880 with two wood+30 chips, 5760 if the opponent is weak. If you can get 4, you have up to 2560, 3040 if wood+30 is added, 6080 if they're weak. You need Flashman to set it up as the move can be blocked or reflected. The kicker, though? You can couple this with the Meteor Combo mentioned above. It can be done after the snakes as it doesn't require holes, and arealocking lasts as long as you're in what would normally be the opponent's back row, and it CAN'T be blocked or counter, meaning it's a satisfying way to finish the opponent off.
Speaking of Blocking or Reflecting, they were also subject to a much needed nerf after 3. Why? With a little practice and some rhythm, you can keep the shield/reflector up basically whenever you're not moving or attacking, leaving precious little opportunity to attack. Reflect was worse, as not only did it block attacks, it reflected ALL damage back to the entire row the reflector was on, meaning if you're trying to attack while the opponent is spamming the shield, you better not be directly in front of them in the very likely chance the attack will simply bounce off. This is why Navi Chips like Flashman were so good as they went through the reflection. This was nerfed later as they added a cooldown time, meaning you had to time your reflects instead of having a continuous shield and also let your opponent opportunities to attack, it also didn't reflect all damage and attacks that went through shields became much more common, getting their own type in "Breaker".
Prism. Any non-breaker attack that hit it will be reflected to all spaces around it. This made it easier to aim attacks as you just had to hit a stationary object if you managed to land the prism in the middle of the opponent's field. Problem is, it also turned attacks that hit a wide area into multi-hit attacks that have all the hits register at the same exact time. The way it worked is if you hit the opponent but the attack also hit the prism, the opponent would take damage from the hit AND the damage from the splash damage reflected on to them. This effectively doubled damage and was used well with LifeSword, DoubleHero (which turned a Game Breaker into a Game Destroyer) and was the basis for Disco-Inferno, a chip combo that utilized the normally average Heat Spread Program Advance. The idea was, as a spreading attack, it would hit the opponent for 300 damage, with another 300 from the prism. Couple that with Grass Stage and you instead deal 600 damage with 600 reflected back, leading to 1200 damage instantly. If the opponent is weak to fire, then the first hit is doubled, having 1200 damage with 600 reflected back, leading to 1800 damage. If you're playing multiplayer, very rarely will you find someone who has more than this, and if you do, there's a high chance they won't be much trouble anyway.