In the earlier games, X can find some well-hidden Street Fighter II-inspired Easter Eggs that let him perform a one-hit kill (or near one-hit kill) attack, thus making the game a whole lot easier after finding them. This is balanced out by the fact that they can only be found in the final boss stages and can only be used at full health, in addition to requiring Street Fighter-like input commands that make them difficult to use in battle. Examples of this are the Hadoken in X1 (which had an added problem of being hard to aim against Sigma) and the Shoryuken in X2.
X3 has the Gold Armor/MAX Hyper Armor, which is a true game breaker. There are four booster chips in the game: armor booster that gives a 75% damage reduction shield when hit, a special weapon that allows you to spam Charge Shots, the power to dash twice before dropping to the ground, or a Healing Factor that would not only restore your health but fill sub-tanks. Thing is, if you take any one of these, you can't take the other chips. BUT! If you don't collect any of those but do find every other item in the game, you can get the secret Hyper Chip early in the fortress, which gives you all four abilities. In other words, you get nearly four times your normal health in addition to a host of abilities that make your character quite unstoppable. The only drawback is that it's not saved in passwords. No wonder the X3 armor is also known as the Hyper Armor...
X1's Storm Tornado. Storm Eagle wasn't exactly hard to defeat if you chose him first. And the weapon he gives you can kill almost every non-boss in one shot, and do a lot of damage to bosses, especially Flame Mammoth.
Homing Torpedo. It shoots out homing missiles, and the energy usage is moderately cheap, making stages a breeze. It doesn't deal that much damage, though.
X2 has Spin Wheel. Fires a sawblade that runs along the ground, hits enemies multiple times, and opens up secret breakable walls and floors. Its charged version is no slouch either, i.e firing a larger sawblade that fires shots all over the screen.
Sonic Slicer fires sets of Razor Wind that cuts through everything and ricochets off walls and ceilings. Its charged version rains crescent beams that deals significant damage, even to minibosses.
X3 had X wielding Zero's beam saber, which was more versatile as it was an upgrade to the charge shot and didn't need a health requirement to fire. With a proper hit, it kills any non-boss enemy instantly and had piercing on top of that (a novelty, but occasionally useful). Any boss enemy would, with a proper hit, be reduced to 20% of their max HP, including the final boss. Combined with the double X-Buster shot that had to be fired prior to the beam saber swing, the result could kill anything with one ten second charge, which is trivially easy to manage.
Spinning Blade. Think Kirby's Cutter power, except you fire 2 at once (one goes up, the other goes down), and that it cuts through multiple enemies. It also deals a lot of damage especially when fired point-blank (3 shots per blade, and can be spammed at point-blank), and it charged version makes you control a giant yo-yo that shreds enemies around you.
X4-X6 gave X the Ultimate Armor, with infinite Nova Strike, while X8 had an upgraded version of this. Unlike X and X2, none of these required you to have full health. None of them were one-hit finishers either, although several were two-hitters.
Ultimate Armor allowed, as previously indicated, infinite Nova Strikes, which in other armors is an attack that affected either the entire screen weakly or one particular area strongly, and then required a lengthy recharge time. The Ultimate Armor allowed Nova Strikes to be chained indefinitely, and they differed greatly as well, turning X into an invincible jet that causes collision damage. In addition to unleashing a devastating attack that can take out any boss in about ten seconds, the Ultimate Armor also allowed X to traverse difficult platforming sections by granting the ability of unimpeded flight.
Those three games also has Zero's "dash-saber cancel" trick: you can cancel any part of his 3-slash combo into a dash, and then the dash into the saber combo. As his first 2 slashes don't trigger Mercy Invincibility on bosses, you can use this trick to shred through their life bars faster than using their weakness weapons. You might ruin it in X5, however, in that W-Shredder's command is the slash button after dashing; it's often advisable to not defeat The Skiver (as you get that technique from him) and just launch the Shuttle without doing so.
Xtreme 2, a game for the Gameboy Color, had a part system like the one in X5. X could equip up to four upgrade parts as long as he didn't have all his armor parts, if he did only two were possible. So as long as your were willing to give up the upgrade for the Street Fighter moves, much weaker than the console versions, you could skip the helmet, which was identical to the X1 version. The best part loadout was Buster +1, Buster +2, Ultimate Buster, and Speed Shot, letting you forget the special weapons and just blast away with a high speed barrage of double-strength charge shots from the X buster. Or you could skip speed shot for the Hyper Dash, which makes you invincible when dashing, and add some defense to your still hideously overpowering offense.
X4 has X's the Plasma Buster Upgrade for the Fourth/Nova Armor that debuts in this game. Fires a large and damaging plasma shot that penetrates everything, and leaves behind TWO balls of energy that do continuous damage to rip up Mooks and bosses to shreds. Ultimate Armor, being based on this armor, also has these two features.
X5 nerfed this by making the plasma shot unable to hit through walls. Now it only left behind one energy ball with the first hit also triggering invulnerability frames in the bosses making it only good against mooks. The head part now only halves the use of special weapons instead of negating it.
X4 also had the Helmet Upgrade, which made all of X's non-charged special weapons cost no ammo. This allows easy spammage of rapid-firing weapons like Ground Hunter, and if it wasn't already overpowered, the Ultimate Armor in X4 also had this.
X5 had the Falcon Armor, which you could get pretty early on if you knew where to look. As the name suggests, this armor gave X flight. Not brief flight, either; flight time was very generous and could be instantly reset simply by landing and taking off again (even a wall cling would suffice), in addition to flight speed being pretty good. Furthermore, the flight also comes with a Battle Aura that can cause damage or outright kill weaker enemies (and some bullets), though you couldn't just plow through everything. It didn't even have any real drawbacks, either (beyond not being able to charge special weapons). In exchange for a trivial reduction in charge shot size and the inability to charge special weapons, X gets a piercing attack that is extremely useful and a screen-cleaning Giga Attack. This was so powerful that it was brutally nerfed in X6, completely eliminating the flight ability, reduced the Buster power, and reduced the Giga Attack damage so much that it was weaker than its Charge Shot, but gave the armor the ability to charge special weapons. Next to the Ultimate Armor, this is probably the biggest Game Breaker in the X series and the biggest nerf that X series ever have during game transition.
The Gaea Armor, while it slows X's dashing speed, has short-ranged shots, prevents him from using any parts and special weapons, and lacks air-dashing, gives him a powerful X-Buster that not only has a destroy some enemy shots, but has a ridiculous charge speed. He can literally spam charge shots like no tomorrow, and these charge shots ignore bosses' Mercy Invincibility frames, allowing him to create a Cycle of Hurting on bosses.
Zero's Black Armor goes from only an aesthetic change to a powerful upgrade. It changes Zero Fragile Speedster status and puts him on equal footing with X by automatically adding the Shock Buffer (reduces damage and stun to half) and the Shot Eraser (allows Z-saber to destroy enemy shots) parts while also allowing Zero to equip the standard 4 parts. It also increases his weapon energy by 50% allowing you to spam Dark Hold and his Giga Attack more consistently.
Firefly Laser normally isn't too game-breaking - it fires a firefly-shaped missile that you can guide in any of the 4 directions which also leaves a trail of light (think TRON) and it has a somewhat large energy usage compared to others. The damage from the missile isn't too great - the real trick is to damage enemies with the trail of light. It's possible to kill a big pack of enemies by careful manipulation of the missile. Its charged version, on the other hand, is pretty straightforward: a continuous big laser that keeps firing for several seconds. Shame it is ridiculously useless on the boss that should be weak to it, Dark Dizzy.
C-Shot also isn't that game-breaking, but its charged version is basically the successor to Rolling Shield's charged version. And Zero gets C-Sword, an attack similar to Kuuenzan in X4 (rolling slash in the air + Double Jump) except more powerful (and effective against Duff McWhalen). The best part is, you get it from Grizzly Slash, the weakest Maverick boss in the game.
X6 parts system. While it has some restrictions (you need to waste hours farming for Nightmare souls) you can customize your character to be a walking death machine or the ultimate tank.
A defensive load for (normal) Zero in X6 is Saber Plus, Energy Saver, D-Barrier, Shock Buffer, and Life Recover. You get enhanced attack power, extra 50% weapon energy, double defense, double the time you're invulnerable after getting hit, and basically a third Sub-Tank.
You can increase your mobility by equipping Speedster, Jumper and Hyper Dash allowing X/Zero to reach otherwise unaccessible places with Heart Tanks and Armor upgrades. The biggest use comes in Gate's Lab as it allows unarmored X/Shadow Armor X to navigate the stage that normally would be a death trap for him.
The Shadow Armor allows X to cling to ceilings and walk in spikes in exchange of the air dash. By forgoing special weapon usage, X's standard shot becomes a spread of ninja stars and his charged shot becomes a charged Z-Saber slash. By adding the previous mobility load onto the armor along with the Power Drive and the Quick Charge you get a Lightning Bruiser that thanks to the parts overcomes the sole weakness of the Shadow Armor.
Zero's Black Armor makes a comeback. This time he gets the Z-Saber Plus part instead of the useless Virus Buster along with the remaining 5 parts load.
The Guard Shell, possibly the most effective weapon in X's reality for Zero, and a candidate for the most effective things in the X series, on par with Ultimate Armor and Falcon Armor. Basically, the shield normally only reflects attack as its name and the game text would imply. However for Zero it has two effects. One is the written effect, and the second effect is multiplying the hit number of close range attack such as the Buster, and the Saber slash combo, and it ignores boss immunity frames. Essentially, you can beat most bosses in the game at 2 seconds using this move.
Yammar Option is among the best "shield" weapons in the entire series. They're basically Attack Drones that protects you from close enemies and increases your firepower as they can also shoot alongside you; better yet, you only consume its energy when you summon the drones, and not when you shoot with them. Its charged effect makes the drones temporarily invincible as well as shooting in multiple different directions as you move around. Better yet, Zero can also use it.
Ensuizan for Zero is essentially the upgraded C-Sword, though it works differently: it can also be done on the ground, and if done in midair, Zero freezes in place as he rolls. He can roll several times at once, dealing high damage to anything around him. Be careful, however, in that when he rolls, he goes out of Mercy Invincibility immediately if you were in that state.
X7; X himself counts. His charged shots have an extremely high damage output, hitting the bosses harder than their intended weakness. Add X7 mechanics (said charge shot homes in AND as big as half of the screen) and you start to understand why X was restricted from the start.
Explosion deals rather immense damage to anything that'll stand still, having great base power and hitting a good four times per shot. What holds it back is its energy usage, so high that you can fire it once without any weapon energy upgrades, and four times if you give them all to a single character. X can make good use of its charged shots, but he'll have to fall back to normal charge shots partway through the battle. Why's it mentioned here? Because on top of its crazy damage output, in the hands of Axl setting Explosion as your active special attack sets you with the almighty Grenade Launcher, a beast of a weapon that essentially shoots charged shots three times as fast as X with the Quick Charge. Though it has a smaller firing range (The X buster can hit things off to the side that the Grenade Launcher can't, and potentially has a longer range), commits you to a jump if you shoot while jumping (like how Simon Belmont jumps), and gets blocked by some things the X Buster goes through (the two transparent blue blocks during the fight with The Professor come to mind), it essentially is a version of Explosion with longer range and infinite ammo, and will shred anything that'll stand still long enough for you to peg it a few times before it moves again. This means it falters against the likes of Flame Hyenard and Vanishing Gungaroo (that he's so jumpy is the ONLY reason you can't annihilate him with his own weapon), you'll have no problem crushing pretty much anything else. Axl himself gets a huge leg up on X if you make good use of the Rapid Fire option so you don't have to hit the attack button for every shot, the Grenade Launcher to compensate for the lack of a charged shot, and his invincible roll to cut through things liable to hit you. While it doesn't make him an absolute superior killer to X, the common impression that he's significantly worse is not true in the least.
Mega Man X: Command Mission had Bait/Stealth Form, which allowed Axl to always be the target of any targeted attack and not take damage. Not a perfect lock, as attacks that hit the whole party would still be able to hit your other characters, but good enough to make many boss battles a lot easier.
And then there's X's Ultimate Armor and Zero's Absolute Zero armors, which increase your attack power to the point that 25K damage can be done with each character. Boss battles will NOT last long enough for the power-ups to eventually wane like they do after a few turns if you use Build Hypers to increase the time the power-ups last. Add SFM Fragment Alpha and you can get a Limit Break out EVERY TURN.
It should be noted, however, that the Ultimate Armor and Absolute Zero are more infinity plus one forms rather than actual game breakers. Both require you to defeat a ridiculously hard Bonus Boss for each one (The Mole Brothers for Absolute Zero, Rafflesia for Ultimate Armor X). The aforementioned SFM Fragment Alpha can only be obtained using Marino's Limit Break on the Final Boss of the game.
X8 has the Σ Blade, the sword Sigma uses in the final battle. Zero becomes a literal walking murder machine with it.
The heroes' Infinity+1 Armors in obtainable in New Game+...
X's Ultimate Armor in this game has the limited Nova Strike, however it will damage them severely (about 97% of their HP gone in a single hit) and charges REALLY fast. It also takes X4's helmet upgrade to the extreme by allowing him to have infinite special weapon energy for both normal AND charged attacks. Also, not only do you get the Plasma Charge shot upgrade back, but the leg upgrades combines the two upgrades you get normally.
Zero's Black Armor fixes his short dashing distance and turns him into a Glass Cannon. Combining this with the aforementioned Σ Blade lets Zero turn enemies into mincemeat.
And finally Axl's White Armor gives him indefinite hovering while firing his weapons, faster dashes, and unlimited transformation time. Feel free to speedrun your way to an AAA/S rank with ease.
Mixing X's Hermes Helmet with the Icarus Buster allows X to spam fully-charged shots (not the Wave Motion Gun although that can also be spammed) like crazy, similar to X5's Gaea Armor only without the crippled dashing speed and retaining the use of his special weapons. These shots ignores their invincibility frames (except when they're using their Desperation Attack) instead of triggering it since it's no longer the guard-breaking attack for bosses.
In Maverick Hunter X, you get to play as Vile. He's got a few breakers up his sleeve, too. One example is the Stubborn Crawler which absolutely MURDERS anything on the ground and on a wall faster than you can blink. In addition, not one, but TWO bosses are weak to it. Then there's the Parasite Sword, which can hit anywhere on the screen if you're positioned properly. It cannot be destroyed, and it's pretty powerful, too. The kicker? There's no limit to how many times you can use ANY of his weapons.
From the vulcan weapons, there is the Distance Needler. Acquired by beating Chill Penguin, this weapon has a slower firing rate than other vulcan weapons. However, it does have a longer range, can pass through just about any enemy (even shielded ones), and deals a pretty good amount of damage per shot. Plus, this weapon can be equipped with both of the ones mentioned above, thereby validating Vile's character as a one-Reploid murder machine.
And from the Missiles, we have the Popcorn Demon, which fires one missile that splits into three. The firing rate on the parent missile makes it very easy to fill half the screen with missiles. If you're playing the UMD version of the game, this will cause your PSP to lag, creating a temporary Bullet Time effect that lasts as long as you keep spamming missiles. This does not work in the PlayStation Network digital re-release.
The game also tweaked some of X's weapons. The arm upgrade you get from Zero is stronger than the normal version, making the buster a good alternative to Rolling Shield against Sigma's second form. Chameleon Sting now immediately fires a spread of moderately powerful lasers instead of a single laser that spreads after moving a distance, on top of its charged version making you fully invincibleand lets you change weapons after activating it. Charged Electric Spark now creates a blast that hits full screen, demolishing any kind of enemy and making it easier to hit Armored Armadillo. Rolling Shield got the most out of it though, as you can switch weapons while the charged barrier is up, so you can have the free hit and fire anything (including the fireball).
If you're holding a charge using the primary fire button, you can use the X-Buster button to execute the fireball without dropping the charge on X's main weapon. You can also remap either button to one of the PSP's shoulder buttons to make it easier to pull this off.