The Holy Water, which damages enemies as it falls to the ground, and when it does, it explodes into a flame that freezes whatever enemies are caught in it and inflicts continuous damage. Pick up a Double or Triple Shot and watch every boss except Dracula go down in mere seconds, including Death. Not even Dracula's final form gets off unscathed, as while it won't deal any damage to him it will freeze him, allowing you to whip away for free. It retains this game-breaking property in Castlevania III, which runs on the same engine, but by Super Castlevania IV, it was nerfed to make it a more balanced subweapon. Definitely a Game Breaker of that time.
It actually does the same thing in SotN but compared to the other game breakers available it doesn't stand out.
Even more so, the Golden Knife combines the abilities to freeze enemies in place and inflict multiple hits with the ability to cover the distance of the entire screen. Naturally, this is the preferred method to kill Dracula, who won't even be able to move in the face of golden stabbity death and will die in a few seconds. It's so unfair that Konami hasn't even bothered trying to nerf the thing; twenty years later, Simon's Quest is to this day the only game the Golden Knife has appeared in.
Laurels turn you temporarily invincible when you use them. As in, you take absolutely zero damage and can just attack with impunity. Knocking back a few laurels and spamming the whip is the other "strategy" to take down Drac. Unlike the Golden Knife, this one did get nerfed (into a simple healing mechanic) and has appeared in later games.
The Sacred Flame is thrown like Holy Water, freezes enemies like Holy Water, and explodes into a vertical column so you can even hit low-flying enemies. Merely jumping and throwing Sacred Flame at Dracula while he's in the process of appearing is the other OTHER way to destroy Drac. (Really, Drac is pathetically easy in Castlevania II no matter what method you use.)
Symphony of the Night
Castlevania: Symphony of the Night was not meant to be a challenge. Most people trying to play the game and actually challenge themselves first have to set up a mental checklist of what they're not going to do. For everyone else, the players who love the idea of mass carnage and annihilating the forces of darkness while they can't even touch you, Konami provided more ways to do so than you can shake a stick at:
Equipping the Alucard Shield and Shield Rod and attacking with both at the same time will allow the character to cast a spell that makes him invincible, causes strikes with the shield to heal him, freezes enemies every time they're struck, and makes it deal such obscene damage (in the region of 255 damage every tenth of a second) it can kill Galamoth and Dracula in short order while making it physically impossible for them to retaliate. It could be thought of as an Infinity+1 Sword, only neither item requires either grinding or going that far out of your way to get.
For extra fun, use the 99 LCK code, and keep Death from taking your stuff. While the Alucard Shield is available fairly late in the game normally, the Shield Rod is available very early. If your shield doesn't get stolen, you get to abuse it on most of the bosses instead of just the last few.
Also, the Shield Rod + Iron Shield combo, which can spam half the screen with significant damage. Notable in that it's available relatively early in the game and there are no random drops involved at all.
The Iron Shield by itself is a Game Breaker, if a more situation one, giving Alucard a few seconds of invincibility every time a projectile hits it.
Ah, the Crissaegrim. It hits four times per swing, each time for about as much damage as any other normal sword which would only hit once, swings as fast as you can press the button, allows you to attack while moving normally while most weapons force you to stop moving when you use them, has triple the horizontal hit radius of any other sword in the game and a vertical one equal to a select few 2-handed weapons that decrease your defense a good amount and, oh yeah, you can hold one in each hand and attack with both at the same time.
You know those Guardians surrounding the clock room? The ones that kill you dead? Easily pwned with this thing. They'll do nothing but stick out their shield, because you will be attacking non stop. Once you get this sword, you can level up insanely fast there. And, if you're a little slow on your button mashing, just get two of them and alternate buttons to be sure.
The Mourneblade just happens to be Crissaegrim's nasty little brother. Same exact stats, except it traded out the four hit bonus for an automatic Soul Steal worth 8 HP each and every time you hit anything. Dodging is officially out of style with this thing: all you have to do is tear forward swinging like Leeroy Jenkins and you'll gain more HP back than you're losing. Even against bosses. Even the final one.
Also worth mentioning is the Rune Sword. It's a huge sword with high attack power that, instead of being swung, flings itself in front of you to a distance of half a screen away and then flies back to you. You can easily strike enemies that are dangerous to get close to or are on the other side of a wall.
It's possible to level up the Murasama to the point where it can kill anything in the game with one hit, though this really is an Infinity+1 Sword since it would take tens of thousands of kills to do it, and by that point you'd have every other Game Breaker on this list available to you.
Poison Cloud makes boss fights ridiculously easy; because by the time you get it you have enough magic power to stay in form long enough to deal massive damage.
At least, it would, if acquiring it didn't require you to first kill Galamoth. The final boss is actually immune to it, as well.
There's also an easy trick to getting a powerful fist weapon, the Jewel Knuckles, much earlier than you're supposed to, which dovetails into another trick that uses them to get yourself to a preposterously high level for that part of the game by repeatedly beating up a defenseless Spike Ball.
Soul Steal. While the pad movement for it is a little awkward at first, it deals horrendous damage to most early bosses and also heals you as part of the bargain, and can be used to kill enemies from safe areas you really have no business being able to attack them from. Each soul siphons 8 HP, and it hits every enemy on the screen about as many times, giving you 8 HP per hit per enemy. It quickly gets less useful as other game breakers become available to you, but it's useful as soon as you have enough max MP to cast it.
Finally, there's Duplicator + Power of Sire/Pentagram/Bat Pentagram. You can only buy one after you've cleared the game and you'll have a hard time gathering up enough cash for it unless you use the Good Bad Bug to amusingly first duplicate your sellable jewels, but once you get one and equip it, the Duplicator gives you an infinite supply of any usable item even if you only had one of it in your inventory, which in case of those 3 items means you can hit everything on the screen instantly for tons of damage.
The Ring of Varda. The Ring (yes, that one) is an accessory granting stat boosts so disgustingly gigantic that the game flatly refuses to drop one at all unless there's already a cleared save game file on your memory card. And yes, you can equip two of them.
Richter's "Hydro Storm" is incredibly broken, allowing you to beat every boss except Galamoth with no real effort.
For Galamoth, he has the dagger item crash, aimed at the head from the upper platform.
Any sword that gives you the teleport dash attack, like the Alucard Sword. With good timing and just a bit of button coordination, you can do this attack over and over again without getting hit.
Maria on the Saturn port. You can beat the whole game and every boss without taking any damage and in record time if you don't mind abusing her invincibility spell. Even without that spell she still kills every boss in seconds, probably a minute for Galamoth. Only a broken Alucard beats her in a speed kill against Galamoth and a few other bosses.
Circle of the Moon
The Uranus + Thunderbird DSS combo. One use and every enemy that isn't a Boss in Mook Clothing (or an actual boss) is just plain dead. Not only is it the strongest attack in the game (besides the Pluto + Black Dog combo), it's a guaranteed hit on every enemy within a radius that extends beyond the screen. As if that weren't enough by itself, you're invincible for about 5 seconds after using it, so you can even use it as a very effective defense.
Pluto + Black Dog can be this with a bit of luck. Yes, you turn into a slow skeleton whose only attack is throwing bones- a One-Hit-Point Wonder one at that. So what makes it a potential game-breaker? Occasionally at random, you'll throw a huge bone. That does an almost-guaranteed 9999 damage. Turning every single enemy and boss in the game into One Hit Point Wonders themselves...
Due to a programming error in the game, if you switch your card combo to something like Mars + Black Dog (even if you don't actually have the cards) just after you activate the summon, its power goes DRASTICALLY up.
And you never really need any cards beyond the Mercury/Salamander, the easiest cards to get in the entire game, because of a similar glitch. If you pause the game when turning on the cards you can change the combination to any combination at all, even if you don't have them. Using this and the Mars+Black Dog glitch requires an irritatingly massive amount of menu handling, though.
Harmony of Dissonance
The Bolt + Bible combo summons a pair of Gradius-style shields to float in front of Juste. On top of blocking projectile shots, it does over 140 damage per hit, and each shield can take about 10 hits.
The Bible itself is the real Game Breaker. In addition to the Bolt combo, there's also the Ice combo, which demolishes more or less anything given sufficient MP. And when you're out of MP, you can use the Bible with no attribute- it's default attack is to spin out from you in ever-expanding circles, essentially hitting everything on the screen up to three times depending on size.
Additionally, the Cross + Wind combo is a great way to dish out the hurt while still retaining the ability to evade attacks. The orbiting crosses have RIDICULOUS range. Also, the Sacred Fist + Wind combo lets you fire a VERY damaging Hadoken for a low amount of MP, and you're invincible while performing this attack.
HoD has yet another quirk that makes it piss easy: you can store up to 99 of any consumable item, as opposed to 9 for later Metroidvanias. With enough money on you, you can stock 99 Potions and Hi-Potions, which makes an already-easy game even easier.
Plus, there's a certain chandelier in the upper left part of the map that drops $400 each time it's broken. Like every chandelier, respawns when the player leaves the room. As if that wasn't enough, it's located right next to the door to the next room.
Chronicles of Sorrow
Aria of Sorrow has the Claimh Solais. It has the highest stats of any weapon, a long reach, swings in a nearly 180 degree arc, and has the holy element, which many of the tough enemies in the game are weak against. Even given that it's the game's Infinity+1 Sword (albeit a remarkably easy to get hold of one), getting a hold of it makes the game insultingly easy.
And a Disc One Nuke, if you obtain it early enough. Also accompanied by the extraordinarily powerful Eversing armor.
You might want to have a dark-element or mundane sword as backup, though. That One Boss resists holy damage, and moves fast enough that Claimh Solais's slower swing is more a hindrance than a help.
The Claimh Solais hits even faster if you're dexterous enough. All you have to do is tap A for a short hop, then tap B to swing the sword; once Soma hits the ground, the swinging animation gets canceled out, and with good enough tempo, one can easily swing the sword at an enemy and retreat at the same time.
And the Valkyrie soul is just as bad, delivering a hideous strike across a huge area for a very reasonable MP cost, and coming much earlier in the game than the Claimh Solais does.
And, of course, it's Holy-element! You can count the number of monsters resistant to Holy on your fingers, whereas at least half the remaining cast of creatures is weak to holy.
The majority of Souls in Aria of Sorrow are pretty powerful, actually: a good example is the Red Minotaur Soul, which summons an axe so big that swinging it clears the entire screen, doing ridiculous damage and killing bosses in a few attacks. So the true Game Breaker of Aria of Sorrow has got to be the Chaos Ring, which lets you spam Souls without using any MP.
Added to this is the fact that standing with your back facing the enemy makes the soul hit TWICE (once when it arcs back to swing, the second when the swing makes contact). For a hefty 150MP, you can still do over 2000 damage in one attack. One can destroy bosses without even replenish their MP.
The Mantle soul, which has the property of swapping HP and MP, can one hit kill many enemies, and costs a mere pittance in MP.
Even better, once you get the Mantle soul, you can use it to do some disgustingly quick power leveling in a hallway in the Arena full of Mantles, Red Minotaurs, and Werejaguars, since the Mantle soul will one-shot the otherwise fairly difficult to kill Red Minotaurs. It's easily possible to gain dozens of levels within an hour or two.
Even if you don't have the Chaos Ring, the Black Panther soul allows you to just charge through most enemies at a high enough speed that you take no damage. And it costs all of two MP per second, making it EXTREMELY spammable.
Except that it's obtainable right at the end, and only if you gain access to the "post-game".
Lightning Doll soul. Has incredible reach, slightly less costly than Valkyrie, deals multiple hits with high power, and you can get it as soon as you have access to the Clock Tower.
The positron rifle from the boss rush mode deserves mention. It hits nearly as hard as the Solais and it multi-hits enemies. It also has the longest range of any weapon, and combined with the Succubus soul, you can decimate any grounded or large enemy without taking a single hit and heal a lot of HP.
Dawn of Sorrow has the Mandragora soul. It simply sends a screaming mandragora out for 30MP. The range is insane, the damage is very good for such a cheap costing soul, and while a little grinding may be needed for it, you can get it as soon as you get into the Garden of Madness. The somewhat rarer and later Great Axe Armour soul costs more and is only a little more powerful, but the range is massive, similar to the Red Minotaur soul but less broken.
There's also the Devil soul, which gives you a huge Strength stat boost in exchange for doing 10 HP of damage each second. The Devil soul is also very easy to get, appearing fairly early in the game. Using it in combination with the Valmanway (see "Crissaegrim" under Symphony, because being broken in just one game apparently wasn't enough), it can beat the game's Boss Rush mode in just over 4 minutes (taking into account the long waiting times to gather the healing orb after every battle).
The Medusa Soul+jumpkick combo: basically, in Dawn of Sorrow, your jumpkicks do more damage the longer you fall before connecting with an enemy, which is normally only useful against Gergoth, since his boss arena is the only one with a long vertical drop. But since the game only counts airtime and not the distance you cover before connecting with an enemy, you can easily use the Medusa soul to freeze yourself in midair mid-kick and do ridiculous amounts of damage to anything below you when you let go. There were several bosses who ate an extremely, extremely damaging boot to the top of the skull.
Curse of Darkness
Curse of Darkness isn't too bad with regards to game-breakery in Hector's story, but playing in Trevor mode—dear god. Spamming the Dagger subweapon crush on some bosses means you can destroy their faces without taking a single hit.
Hector can be pretty break-y too if you're prone to experimenting with weapons. Using the replica of Isaac's spear against him in the second battle is almost unfair because your combo is faster than his recovery, and if you keep hitting him from behind (Foe Yay abounds with those two), you can get him stuck in perpetual recovery stagger until he switches modes, or until one of the IDs lambasts you.
The End would probably be broken as hell, if he wasn't such a bottomless heart-munching beastie and prone to attacking everything large and moving.
Trevor's speed-kick combo also bears mentioning. Massive damage, tons of hits, can track enemies. Many a boss have had their bodies shattered by this thing. Makes you wonder why he even bothers with the whip.
The Stellar Sword's special move HITS EVERY ENEMY ON THE SCREEN. As in, Jonathan creates shadow clones to teleport and slash at each enemy simultaneously. Also, it has the Holy element, and it's got a good amount of power. But would you expect any less from Maxim's preferred weapon?
The 1000 Blades Dual Crash is probably the worst example, though, because you get it so early in the game. It's not actually one thousand blades, just 48 (or 51 if your partner is on-screen), but even that will kill pretty much anything in a few strikes, and since each stuns separately the enemy is quite screwed. The high hit count also makes it perfect against Final Guard, a Boss in Mook Clothing who has 50 HP... but with a DEF so high that it takes only 1 HP of damage per hit from everything.
The Ancient Armor makes Level 1 runs trivial. It turns you into a Ten Hit Point Wonder, which doesn't seem like a very good deal until you realize that ten hits is far more than bosses like Dracula, Death, or Whip's Memory would take to kill you in a fair fight.
And that's not taking into account the fact that it allows you to use potions and other healing items to increase the total amount of hits you can take: Take 9 hits, use a potion, take 9 hits, use a potion...
Portrait of Ruin has another massive gamebreaker in the inventory, where tapping the stylus at a certain place will always select a potion and allow you to use it... even if you don't even have a potion, so you can at any time use any amount of potions at will. This makes everything even more trivial, as any damage taken can always be instantly fully recovered. With proper usage of potions it's nearly impossible to die.
Order Of Ecclesia
In the proud tradition of CastlevaniaGame Breakers, Castlevania: Order of Ecclesia included a storyline-sanctioned breaker in the form of Dominus Agony. With it active you get massive stat boosts, but lose 66 HP per second. Combined with the Death Ring (even more massive stat boosts in exchange for becoming a One-Hit-Point Wonder), you can drain bosses' hit points at a ridiculous rate. It can kill Dracula in about two hits!
If you add in another Death Ring (you're already going to die in one hit anyway thanks to the first Death Ring so why not get another obscene stat boost) and equip the Nitesco Sword (Nitesco glyph + any weapon glyph), you're basically destroying half the screen every time you attack. Throw in a Queen of Hearts (reduces the heart consumption of Glyph Unions) and you're getting twice as much overkill as you were before.
It's made even more broken by the fact that it consumes less hearts than the de facto strongest union (Light + Dark), and is only slightly less powerful.
The only trade-off is Dominus Agony is draining your HP at a ridiculous rate, so you're better off leaving that inactive except against bosses. Not that you really need it. Just the Nitesco Sword and a single Death Ring is more than adequate for 99% of the game.
The Arma Custos glyph (also a mandatory acquisition) increases Shanoa's STR as her health gets lower. Combine with all of the above (in place of Dominus Agony) and a Judgment Ring (increases damage dealt by combo attacks) in place of the Death Ring, and Shanoa is still breaking the faces of bosses and mooks alike before they know what's happening.
Order of Ecclesia also has a "tape down buttons" trick that can lead to maxing out all your elemental experience points, simply by repeatedly absorbing glyphs from the floating goat-like spellcaster foes. It involves an accessory that completely nullifies Shanoa's reaction to getting hit (apart from the damage, of course) with high enough defense that the enemies don't do more than 1 damage to her when they fly into her, combined with a glyph that regenerates Shanoa's health. The goat-dudes can hover right over her face as much as they want, but since her defense is high enough, the regeneration glyph heals her faster than they can deplete her health. As long as you have Up taped down, you'll keep absoring their glyphs, preventing them from using their attacks with each absorbed glyph boosting all of her elemental attributes by 1 up to a maximum of 65535, at which point all of her attacks will do 3 times the normal amount of damage.
When playing as Albus you have access to the teleport ability, which you use by touching an empty spot on the screen. It's most useful outside of boss fights, as you can skip platforming segments with it.
Harmony of Despair
The Valmanway. Just...the Valmanway. Remember the Crissaegrim from SOTN? The Valmanway is the very same weapon, but weaker. However, it slashes 4 times per second, making 14 damage into 56 damage. With stat increasing items it can deal a maximum of 35+ damage per hit. This makes it a staple for Somas and Alucards on Hard mode. Needless to say, a lot of people who believe the game is meant to be experienced "fairly" abhor the existence of this sword.
The Yasutsuna, when wielded by Alucard, was very similar to the Valmanway before the patch, which changed its hit rate. It is still useful, however.
Valmanway +1 in Chapter 10 made this worse, despite the fact it's just a slightly stronger Valmanway, by 6 points of attack. So that 20 attack is now 80 attack, and stat increases mean it can deal 45+ damage per hit.
Soma himself is a complete game breaker with two of these, the succubus soul, the astral ring, and on some bosses the Medusa Head soul. This lets Soma hang in mid air, spam his Valmanways, and recover the health lost by the Medusa Head soul. Other than that, Soma can freely spam whatever soul he wants so long as he goes and hits a few enemies.
Before the patch, Charlotte had a special accessory combination that essentially gave her infinite health to both herself and her teammates. All you needed was her Heal spell and an Astral Ring that allows her to cast spells using HP instead of MP: this allowed her to heal herself for more HP than the spell cost to cast, and naturally it healed any of her teammates for the full amount. Adding a Skull Ring turns her into what fans dubbed "Super Saiyan Charlotte", due to the convinient status increases it gives her as well as a yellow glow from being cursed that reduces her MP to 0, which didn't matter to her since she didn't use MP to cast spells with this setup.
There is a glitch in Chapter 10 involving Soma's Puppet Master soul which allows you to completely bypass 3 of the 6 minibosses and go straight through the lift to the Clock Tower into Dracula's Tower, which is extremely handy considering the stage is a mostly faithful reproduction of the entire Castlevania I and is thus a Marathon Level with many extremely useful items you can get from killing R. The Count repeatedly.
Soma's Yorick soul also lets you skip sections of some stages. It can be used in Chapters 3 and 4 to get to the bosses very quickly.
It's recently been learned that Richter can perform the same glitch in Chapter 10 using his spin kick ability. Everyone else with a jumpkick can apparently do it as well, but it requires a ton of precision and is a lot harder to do and every time you mistime it, you're guaranteed to be sent at the very bottom of the elevator shaft and forced to spend another 20 seconds to ride one back up to try again.
There's YET another new Soma-only glitch that allows him to reach Dracula's room from the very beginning of the level using the same overall principle, but it's much harder to pull off and requires ridiculous amounts of precision and speed to do.
IGA may have declared Castlevania Legends to be Canon Discontinuity, but that didn't stop Sonia Belmont from being able to walk all over the forces of darkness. First off, there's Burning Mode. It grants total immunity/invincibility (for a short period of time). The only drawback is that it's only usable once per level and life, but Sonia has this power from the start. Couple this with her Soul Weapons (abilities gained from bosses; most likely to make up for the lack of usable subweapons in this game), which can freeze time, restore life, and fire waves from the Vampire Killer (among other powers), and you've got one cakewalk of a game.
And then it gets worse if your opponent figures out that his counter (which itself leaves you open for a basic combo) instantly gives Aeon a charge of his click, it means that he can play the battle defensively by spamming counters and then unleash hell upon you with the charge of his choice.