The only actual restriction on "superfighters" like the X-02 Wyvern, ADF-01 FALKEN, or CFA-44 Nosferatu is price to buy and unlock method; there are comparatively powerful late-game planes that are balanced in both ways, but the Su-37 is an exception.
The game's Infinity +1 Sword, the XFA-27, can launch four missiles in one go. While mild compared to pretty much every other AC 'Breaker (just look at the entries below), it does make ground attack much quicker, especially since missiles are available for use again as soon as a hit is registered.
The Orbital Satellite Laser arguably competes for the title of most overpowered weapon in the series because it can be used by any aircraft in the game. Though it's a Bragging Rights Reward because it's unlocked after getting A Ranks on Hard, a cheat exists where you can use it before having unlocked it.
The Quick Maneuver Air-to-Air Missiles (QAAMs), which are true "fire and forget" guaranteed air-to-air kills. While these were toned down in subsequent games (up until Ace Combat: Assault Horizon, where they were buffed back to their previous glory/infamy), in 6, they're usually considered a "n00b" weapon that online can only be best countered by... yep, bringing your own.
The final mission is against eight of the toughest enemies in the game. It is intended to last at least thirty seconds for even the best players. However, take any plane that uses the XLAA, and you can one-shot-kill four of them from beyond their range, then perform a loop and one-shot the survivors. The game literally breaks if you do it right, as since you are not meant to beat them that fast the speech cannot keep up with the speed of the battle, so you are basically waiting for up to a minute after the battle is over.
On the plus side, after the battle your wingmen have quite a few thoughts to share before attacking the SOLG. Break the game and you can easily listen to all of it rather than be interrupted by the attack if you went the long/hard way before.
The mission "Demons of Razgriz" can be difficult on higher levels as the Hrimfaxi's anti-air capabilities are quite effective. However, the mission can easily be beaten by any aircraft with the LASM, as you can hit the Hrimfaxi without risking damage. The only reason the pilot wouldn't want to end the mission in a few minutes is that winning too quickly means you won't get enough points for the S Rank from shooting down the accompanying SLUAVs that are launched for defensive purposes.
The FALKEN's Tactical Laser System is brutal once you get your hands on it. Though requiring skill, it can insta-kill multiple fighters in one charge. You can even break the second half of the aforementioned final mission with it — the aforementioned SOLG, a giant satellite falling to Earth, can literally be cut in half with a single TLS shot.
5's version of the MiG-1.44 is this in pretty much any Multirole mission and most ground attack missions. It comes equipped with eighteen large unguided bombs, almost twice the amount of the A-6E Intruder, the only other plane equipped with them. At the same time, it also has a whopping 96 Mobility rating that allows it to dogfight with the best of them and a crap ton of standard missiles. This plane can rip through something like the Hrimfaxi with ease, without compromising the ability to defend itself in the air. The only downside is that it's unlocked only on the last two missions of the game.
The F/A-18 family in this game is considered to be one of the best plane families for a one plane/one family run on account of its great special weapon loadout (including the XMAA), great maneuverability, and availability in carrier plane-only missions.
There's a mild version of this in the Su-37 Terminator, which has a huge standard missile load, can equip the second-best missiles and best bomb in the game, and can turn on a freaking dime — and unlike in other games, where it's a late-game fighter at best, the Su-37 available as soon as you beat Mission 6. There's little reason to fly anything else once you can afford it. The Eurofighter Typhoon is available even earlier, after Mission 3, but only on the Mercenary Ace path (which is supposed to be harder than the Soldier or Knight paths), and it doesn't carry the Fuel Air Explosive (FAE) Bomb, which allows one to finish the Hresvelgr in three hits and Avalon (one of the infamous tunnel missions in the series) in a single fly-through.
Zero also has a much less mild version of this in the ADFX-02 Morgan, which has a weapon that can finish some entire missions with a single shot. Then again, it is the final boss' plane, and he's already demonstrated its nastiness on you.
If you have save data from previous games, one of the two balancing measures for the X-02 Wyvern and ADF-01 FALKEN is removed: the FALKEN's requirements are reduced from having to get all three Supreme style medals (beat the game one time each with the Ace Style gauge as far as it will go in either direction for Knight and Mercenary and in the direct center for Soldier) to simply beating the game one time each with every Ace Style, and the Wyvern's are reduced from beating the game twice with different Ace Styles to simply completing the first mission.
The CFA-44 Nosferatu's ADMMs can kill up to twelve targets and has a ridiculously long lock range, and the EML can deal severe damage if not insta-kill anything. Oh, it also can be bought as special DLC, so you can start with it from the get go.
The CFA-44 is an Infinity +1 Sword on its own, but its main flaw is that it stalls easily. This is to balance for the fact it's an overpowered plane. However, if you purchase the Razgriz paint, the CFA-44 loses this flaw, essentially becoming a perfect, flawless aircraft. An Infinity +2 Sword, if you will.
The Su-47 Miki-EX is this in multiplayer, thanks to reality-defying maneuvers and a metric ton of QAAMs, allowing thousands of points to be racked up with ease. Yes, like the CFA-44, the Miki-EX is DLC and can be made available from the start.
In fact, any of the The Idolmaster aircraft can be this because of how much ammo they can carry, in addition to any unique stats and abilities.
Any of the energy-based weapons (PLSL, TLS, EML) can be very effective simply because the A.I. has no idea how to deal with long-range weapons and very little idea how to avoid direct-fire weapons. While some travel time has to be accounted for with any weapon except the TLS, this makes them very effective against the A.I., and their high power makes them very useful in multiplayer. Special note, however, goes to the PLSL, noted below.
This game marks the return of the Pulse Laser (PLSL) last seen in Electrosphere. Unlike the TLS and EML, the PLSL has no reload time and can be rapid-fired much like your standard machine guns or the MGP. It's also easier to aim and much more plentiful when it comes to ammunition, with the Su-57 having a whopping 650 laser bursts as its maximum capacity. Furthermore, the laser's range and attack power have been significantly improved from AC3 (and can be further enhanced with parts), making it a great answer to ground and aerial threats alike. This means players can snipe peskier targets* such as the Arsenal Bird in Missions 3 and 19, bombers in Missions 5 and 12, the naval fleets and platforms in Mission 11, etc. from afar with little difficulty or risk of being fired upon. A skilled enough player can even use it to down opposing aircraft long before they're within range of missiles or special weaponry — a godsend in a No-Damage Run. If that wasn't enough, it's fairly easy to obtain early on in the campaign, being assigned to the MiG-31B Foxhound and F-15C Eagle. The only significant drawback is that the PLSL is unable to pierce through cloud cover, limiting its effectiveness in missions and multiplayer maps with heavy weather conditions (or even ones with partial cover like Fort Grays Island).
While the Pulse Laser's unmatched versatility and ammo capacity make it unquestionably the best weapon for singleplayer, the Electromagnetic Launcher (EML) is the king of multiplayer. The Level 2 parts exclusive to multiplayer upgrade the EML's hitbox size until it's larger than any aircraft and its damage until it can either instantly destroy or critically damage an enemy aircraft, even if they're equipped with a Level 2 armour part. The result is one of the most powerful special weapons in multiplayer.
The Integrated Electronic Warfare System (IEWS) completely destroys multiplayer. It makes it much harder for enemies to lock missiles and reduces the accuracy of missiles in flight, while also enhancing your allies' missiles' speed, homing, range, and lock-on time. Even in single-player, it's very powerful against bosses and other highly evasive enemies as it offers most of the same buffs as the Enchanter electronic warfare planes in Unexpected Visitor, save for not giving the damage buff. Originally it was somewhat limited by only being on the ADFX-01 Morgan, which has rather uninspiring performance among series-original aircraft and a high point cost in multiplayer. However, the 25th Anniversary Original Aircraft Pack DLC added the XFA-27 and CFA-44 Nosferatu, which also have IEWS and much more impressive performance than the ADFX-01.
The QAAM when properly upgraded and on a good plane like the F-22A is as broken as ever, though the EML on the Strike Wyvern and Nosferatu and the IEWS on the Morgan, XFA-27, and Nosferatu do curb it down a notch. If no superplanes are in a match, though, a properly built and played QAAM Raptor is a very tough opponent without PLSL.
The CFA-44 Nosferatu takes what is arguably the most dominant multiplayer weapon, the EML, and mounts two of them, each with separate cooldowns. This allows it to take an immediate follow-up shot if it misses the first one. It can also instead take IEWS to be an extremely powerful support player in Team Deathmatches.
The Darkstar from the Top Gun: Maverick DLC may not be capable of reaching Mach 10, but it is still the fastest plane in the game, even faster than the MiG-31B. Its sheer speed (exceeding Mach 5), combined with PLSL both as a substitute to guns and as a special weapon option, allows it to zoom past and fire lasers into the central furball, and then outrun any missiles lobbed its way before lining up another run (except for HVAAs — those can keep up with Darkstars and are often a must if the lobby becomes filled with them).
, which has a huge standard missile load, can equip the second-best missiles and best bomb in the game, and can turn on a freaking dime — and unlike in other games, where it's a late-game fighter at best, the Su-37 available as soon as you beat Mission 6. There's little reason to fly anything else once you can afford it. The Eurofighter Typhoon is available even earlier, after Mission 3, but only on the Mercenary Ace path (which is supposed to be harder than the Soldier or Knight paths), and it doesn't carry the Fuel Air Explosive (FAE) Bomb, which allows one to finish the Hresvelgr in three hits and Avalon (one of the infamous tunnel missions in the series) in a single fly-through.