The games feature a Singularity Engine Time Accelerator ship add-on that allows you to speed up travel times via time compression.note Justified in-game as a Time Dilation device. Setting SETA to 10x causes the game to prioritize where it uses system resources. In other words, it will run AI routines slower (making them moreidiotic), meaning that if the player hides in, say, a station while SETA is on 10x, in a combat mission, almost all enemy ships will plow into the station and destroy themselves because their AI doesn't realize there's a station in their flight path. When your system is really being taxed, the AI will also stop firing (and in extreme cases, stop moving entirely) at you when SETA is on 10x.
Nividium mining in Terran Conflict and previous games. Nividium is an extremely valuable, rare mineral that can only be mined by breaking up asteroids and picking up the debris in ships. Normally, doing this will result in the nividium being depleted. However, ordering your ships to pick up the debris when you are not in the same sector results in the game basically "faking" your ships picking up the debris — they'll fly around in the debris fields, and the game will cheat in nividium into their cargo bay, in order to save processing power. The game does not get rid of the nividium that the ship is pretending to pick up, meaning that doing this results in effectively infinite nividium. Mine the debris for an hour, load it all into one ship, sell it for 30 to 50 million credits at a shipyard, rinse and repeat till you have billions of credits. This exploit was fixed in Albion Prelude.
The Bala Gi patch for X3: Reunion introduced the M3+ heavy fighter and M7 frigate class ships, respectively a fighter and capital ship that vastly outmaneuver and outgun all other ships of their type. Arguably a necessary evil, as the normal ships are all holdouts from the original 1999 game and generally suck.
The Hyperion M7. While it's by no means a God Ship, and easy to kill when piloted by the AI, it becomes a one-size-fits-all pocket destroyer when flown by the player. Its superior weaponry lets it kill any other ship of its size, and its speed makes it fairly easy to dodge fire from ships several times its size — all the while pumping laser blasts in their hull. Bigger ships are still useful for massive fleet actions, but in solo play once you get that it becomes redundant to get anything else at all. It returns in Terran Conflict, albeit nerfed to have its firepower and shielding being more in-line with corvettes; it was also reclassified as a M6 Corvette as the newly introduced M7 Frigates are proper capital ships. Still ends up being the best corvette after the (much more difficult to acquire) Springblossom, though.
The Phased Shockwave Generator is an immensely powerful Area of Effect weapon that does wonders on both fighters and capital ships. Add to the fact that it can be mounted on most M3 fighters and you got yourself a weapon that can rack up your kill count and combat ranking very quickly. This weapon became nerfed to being only compatible with capital ships in Terran Conflict and Albion Prelude.
X3: Terran Conflict
The Springblossom corvette in X3: Terran Conflict. It's a corvette which has the firepower of a Prototype M6, can outrun most fighter craft, has the cargo bay of a small transporter, and can use the Prototype Starburst Shockwave Cannon, which is a frigate-grade flak weapon. The only disadvantage to it are the rarity of the weapons it uses (you can't build factories for Aldrin weapons), and the fact that the insane speed will mean you're probably going to end up ramming into enemies and dying, a lot. Its little brother, the Spitfyre interceptor, can outrun most scout ships, has the firepower of a prototype M3 (or potentially an M6, if you use Matter/Antimatter Launchers), and the shielding of a M3 Raider.
The speed of the Springblossom and Spitfyre is somewhat justified by fluff. Their native Aldrin has no jumpgates or trans-orbital accelerators to quickly get ships from one planet or system to another, so they put all their design efforts into getting as much power out of the engines as possible to reduce travel time. (The Spitfyre is basically a cockpit with big f***ing engines attached.) When you combine that with the strong shields and decent weapons of Terran ships, however, you get a Game Breaker. Though when flown by the AI, the Spitfyre is somewhat balanced by the game's navigation difficulties since it has a tendency to crash into things.
It's even lampshaded on the X3 wiki. Once Aldrin was reunited with the X-Universe, the other races were amazed at how much the Spitfyre and Springblossom outperformed their own M3 and M6 designs.
M7Ms also make it much easier to board capital ships, which can then be sold for profitsss. A capital ship will net you anywhere from 20 to 50 million credits, making it really easy to get lots of money fast.
Due to a typo in the data for its anticapital weapon, the ATF Skirnir is a Game Breaker even by M7M standards. Like all M7Ms, it has eight missile tubes. Its primary weapon is the Shadow missile, which has eight warheads. Each warhead does 755 megajoules of damage. The toughest ship to appear in the vanilla game has 12 gigajoules of shielding. Then activate the Missile Barrage command with the Shadow for only one round. Do the math. Albion Prelude fixed the Shadow so Argon characters don't get their heads blown off the minute a Skirnir shows up.
Meanwhile, its Ghoul missiles, while time-consuming to obtain since there's no purchasable factories for them, are lethal against fighters given that such swarm missiles are nearly impossible to shoot down, and the Ghoul will even retarget if there are warheads left over.
The Terran M1 Tokyo was formerly considered a mild Game Breaker by the standards of its class, because unlike most full-size carriers its weapons generator was powerful enough for it to fire its anti-capital guns indefinitely. This ability was removed in the 3.0 patch. However, the ship's asymmetrical design causes a weird problem that can't be fixed as easily: the "center" of the ship that enemies will target is essentially floating in empty space ahead of the flying-saucer-bridge-thingy. This causes most weapons to miss the ship entirely unless manually aimed by the player, or when the weapons are fired at the Tokyo from the left or right sides.
The Split M7 Panther is sometimes considered one, because in addition to 32 fighter berths it has identical performance and power generation characteristics to its fighter-less sister design the Tiger. It received a nerf in Albion Prelude. It still has more fighter berths than some carriers, but its weapons have been weakened to compensate. Naturally this provoked outcry from Panther-lovers.
When Terran Conflict first came out, missions had very high payouts. A "Very Hard" patrol mission would net you about 20 million (enough for a TL or a fully decked out corvette). Said patrol missions could be stacked, leading to payouts of several hundred million, for about an hour's work — several thousand times faster than any other method of money making, save possibly for nividium mining.
The Terran M2 Osaka is tied with the ATF M2 Tyr as the thickest-shielded ship in the game, and unlike the Tyr actually has enough cargo space to carry a full complement of anticapital weapons and shields simultaneously. It has enough reactor and battery capacity to, in the 24xPSP + 24xSSC configuration, continuously fire every weapon on the ship simultaneously for 75 seconds before running out of reserve power. Its top speed is about average for the class. Still, it doesn't get the hate Game Breakers usually do, if only because a force of Osakas is about the only reliable way to permanently blockade sectors against those thrice-damned Xenon Q's. (It and the Tyr are the only ships guaranteed to survive the first OOS exchange of fire with a Q, which they will then promptly curbstomp.)
X3: Albion Prelude
Albion Prelude's Stock Market feature introduced a whole slew of ways to make absurd amounts of money in ridiculously short amounts of time. The Stock Market will determine the "Supply" and the amount of stock shares (2 stocks per item) of a good by how many of the goods are present in the Stock Market's domain. This includes inside cargo bays, such as on your ship. So if you warp into a sector with your cargo bay full of thousands of tons of Ore, the price drops off dramatically, and thousands of stock shares will suddenly become available. Buy up the stocks at rock bottom prices, drop off a Satellite (to deal with the stock market remotely), leave the sector, watch the price of of the stocks skyrocket, and then sell your thousands of stocks. Nividium is a prime candidate for this: while it's rare to find nividium rocks, the ware is also extremely valuable and typically the amount of nividium present in stations and non-player ships is measured in single digits. 700 units of nividium mined and placed in a cargo bay costs about 12 million credits to buy, but when you sell the stocks back, you get about 30 million credits back. Warping between Scale Plate Green and Nyana's Hideout, buying stocks in the current sector and selling it when you move to the next, allows you to make a hundred million credits (normally hours and hours of play time) in about two minutes. Filling a Mammoth's cargo bay with Nividium will net you a couple billion credits when you do just one stock run.
Though there is one downside to using the stock market. The stock prices don't have any effect on the economy, and the game's economics engine deletes under-performing stations as a matter of course to save processor power. This means that the trade-grinding you didn't do didn't stimulate the economy, so it's entirely possible the things you meant to buy will no longer be available.