An ace custom is a piece of technology — a Humongous Mecha, Space Fighter, a particular weapon, or something similar — that differs from the normal model due to being tweaked in order to better fit its user. Said users are typically Ace Pilots, and their equipment is customized to their specifications — thus, ace custom. The extensiveness of this customization varies widely, from simply giving it a new paint job to actually having a completely unique machine built from scratch. Closely related to the Super Prototype, an ace custom will usually be significantly more powerful than a standard model.
Normally, ace customs reflect their user's combat style — a pilot preferring high-speed combat will boost their engine output or add additional thrusters, a gunslinger with Improbable Aiming Skills will add a big gun with scopes and bipods for increased accuracy, etc. This may allow them to be Weak, but Skilled, if their equipment doesn't have as much raw power as their opponent's, but it's so tuned to their strengths that they can compete anyway. It may become so much so that it becomes a case of Only I Can Make It Go.
Everyone else just has to settle for the Mook Mobile.
∀ Gundam has an ironic twist, with Gavan Gooney choosing the most distinctive of the Borjarnon units unearthed. Fans would recognize the Borjarnon as the Zaku II from Mobile Suit Gundam, while Gavan's is a Zaku I.
The Astray prototypes from Mobile Suit Gundam Seed Astray on the other hand... Red Frame, Blue Frame, and Gold Frame have been so heavily customized by their respective pilots that they barely resemble the original model anymore (especially the Gold Frame, which, among other things, actually has the Blitz Gundam's right arm that Kira chopped off!)
Taken to the logical extreme in Gundam Build Fighters, as to win, one must have a custom model of a suit that likely was already an Ace Custom or Super Prototype in the above series' to even have a chance of winning for the most part. Just piloting a repainted Ace Custom will result in a broken model - just ask that one Legend Gundam that got turned into mincemeat by the Kampfer Amazing.
Super Dimension Fortress Macross has Maximilian Jenius and his wife, Zentradi defector, Milia Fallyna, being granted the privilege to give their mechas distinctive paint jobs for being the top pilots of the force.
There are two ace custom Valkyries, Hikaru's that has a triangular visor, two guns and a "chin" (VF-1J), and the Skull-1, with FOUR guns on the head (VF-1S). Both have custom paint jobs as well - Hikaru's is painted in red stripes, the Skull-1 is painted with black and yellow ones, with prominent skull-and-crossbones.
Code Geass features several ace customs. The Knights of the Round are an elite, 12-person ace custom squad piloting highly personal, one-of-a-kind mechs, including the Transforming Mecha Tristan. The Japanese side has the Guren, the Shinkiro and the Zangetsu as their ace customs.
The Shinkirou is an interesting case; it's the Ace Custom of Lelouch, who's actually pretty bad at straight combat. However, it's tailored specifically to his strengths, wielding a scattering laser cannon and a Beehive Barrier that require a mind as quick as his to coordinate properly; in his hands it can single-handedly clear an entire battlefield in moments.
In Magical Girl Lyrical Nanoha, the TSAB's mooks use relatively standardized Storage Devices (spear-like ones for the melee-oriented Belkan style and staves for the range-oriented Mid-Childian style), but their elites are granted much rarer Intelligent Devices with more sophisticated AIs and forms tailored to their areas of specialization.
Notably, Chrono Harlaown's Durandal is a mere Storage Device, which he prefers because it processes magic more quickly than an Intelligent Device, and he is not tempted to grow dependent on its AI.
Teana and Subaru from StrikerS each use a Storage Device rather than an Intelligent Device, but their Devices are both scratch-built and therefore still count as Ace Customs.
Black Lagoon: Revy's Weapon of Choice is a pair of customized Beretta 92 series pistols with, among other things, extended barrels. She calls them the 9mm Sword Cutlass.
In Martian Successor Nadesico, Akatsuki Nagare has his own Aestivalis when he joins the crew, outfitted with a better engine and armor material than the normal Aestis. When you're the CEO of Nergal, you can do these things.
The main girls of Infinite Stratos have their IS Suits customized to their fighting style. For example, Houki's IS takes advantage of her life-long kendo training, being a high-speed melee-focused suit wielding a katana. Charlotte's IS Suit is technically obsolete, due to being 2nd-generation (everyone else's Suits are 3rd-generation), but she's added several modifications (such as being able to dual-wield guns and being able to store and summon more weapons), which comes as a great surprise to her opponents (much like her true gender initially being a secret).
Technically, the Black Getter Robo of Getter Robo Armageddon is this, being a heavily modified Getter Robo.
The Millennium Falcon from Star Wars doesn't look like a whole lot, but due to both Han's piloting skills and special modifications he built into it himself it is considered the fastest ship in the galaxy. In addition it is a bit more heavily armed than average. That doesn't stop it from occasionally acting up as The Alleged Car would. Also, Slave I and Vader's TIE Advanced fighter.
Porco Rosso's plane from "Porco Rosso" is the only one of its kind, as it was considered too dangerous to fly. Porco says (paraphrased) that while it's a nightmare to get in the air he thinks there's no better plane once it's flying.
Mack Maloney's Wingman series features numerous ace customs — the series is set in a post-WW 3Divided States of America, so the combat aircraft flown by many of the protagonists (including the main character, Hawk Hunter) are practically their personal property. The most prominent is Hawk's F-16 (possibly the last one in the world), which gets upgraded midway through to the F-16XL variant.
Paul Naughton's Valkyrie Into The Heavens series features a number of different ace customs. Set in the future, the starships that the protagonists use are cutting edge prototypes and the only ones in existence until the designers standardize the designs.
A two-episode story on Space: Above and Beyond featured "Chiggy Von Richthofen", a enemy Ace capable of wiping entire squadrons of Earth fighters by itself. "Chiggy" flew a special version of the standard Chig fighter that had some kind of stealth technology making it invisible to Earth detection and targeting systems, was invulnerable to cannon fire, and in general displayed greater speed and maneuverability than any other fighter in the show. Also, its nose was painted with a human skull and the phrase "Abandon All Hope" (in English, so his opponents could read it).
In the end, Chiggy gets taken out by a Designer Baby human ace flying a standard-issue Hammerhead fighter with a single missile.
Doctor Who: Inverted (sort of) with the TARDIS, which was specifically selected by the Doctor out of a number of TARDISes because this particular one had a broken navigation system to begin with, and which has developed many more faults since. But these idiosyncracies fit the Doctor's style perfectly, and so he manages to take what should be The Alleged Car and use it as a genuine ace vehicle.
Done in the BattleTech animated series, and in the world setting for the original game as well. Many "ace" Mechwarriors have unique mech variants, such as Justin Allard/Kai Allard-Liao's Yen-Lo-Wang Centurion, or Natasha Kerensky's "Widowmaker" Daishi.
OmniMechs, with swappable hardpoints, were pretty much specifically invented to allow players to build their own ace customs. In fact, there's nothing stopping any player or character with a Daishi to field the Victor, Hohiro, and Widowmaker configurations themselves.
Subverted and then reinforced by the case of Yen Lo Wang. After nailing an astonishing five kills in his Trial of Position with Wolf's Dragoons, Ace pilot Kai Allard-Liao is offered a Dire Wolf (Daishi) Omni Mech by the Dragoons. It's an unquestionably superior off-the-shelf machine. He refuses it because he feels he owes it to his father to pilot their family mech, Yen Lo Wang. No one believes for an instant that the upgraded but antiquated holds a candle to the clantech behemoth they've offered him, so it's subverted. The trope is then reinforced because the Dragoon instead completely overhaul Yen Lo Wang with clan technology, making it infinitely superior to all others of its type and most mechs in the Inner Sphere.
Because many 'mechs are very, very old, many warriors with older machines have had to replace too-expensive or outright lost-and-forgotten older systems and weapons with substitutes, which end up creating a sort of ad-hoc Ace Custom out of necessity. The quality of these machines varies quite a bit depending on what components were being replaced by what. This can lead to very scary Battlemechs being paper tigers because their powerful systems and weapons have been replaced or stripped, or it can lead to an awful surprise when a disreputable "white elephant" or otherwise unassuming design suddenly whips out a BFG.
The Strategic Operations supplement offers players the ability to do aftermarket refits, and can significantly revamp a mech in so doing. There's also a quirks system which can represent both common features of a design or specialty add-ons to a particular mech.
Warhammer 40,000's own Maximillion Weissman had his own custom version of a Baneblade built. Functionally however it was near-identical to a normal baneblade, except for Weissman's superior crew which gave it slightly superior stats. Commissar Yarrick also has his own version of the Baneblade, the Fortress of Arrogance, which does have more noticeable modification. It was presented to him as a gift from the Adeptus Mechanicus.
Tau Commander Farsight has his own Battlesuit customized with the unique Dawnblade he found, subsequently making him the only tau unit dangerously competent in close combat, which suited his bloodthirsty tendencies.
Asdrubael Vect can get a custom ravager/raider called the Dais of Destruction that packs 3 dark lances, armor 13 all sides, and a transport capacity of 10, meaning it will ruin someone's day quite nicely (if a more than a little expensive at 240 points for Vect and another 200 for the Dais)
Getting technical, any of the titular mecha from Armored Core are ace customs. Each one is a specially built engine of destruction that accommodates the pilot's fighting style and can wield an astounding array of weaponry. Also, when Armored Core 4 appeared, mass production models of Cores appeared as mooks— Normals. The newer NEXTS now being the weapon of choice for Ravens, err Lynx.
Xenogears uses this trope a lot, especially in the first disc. Named characters' Gears tend to be unique models custom-built for the pilot, while mass-produced Gears are used by Mooks. However, in the latter half of the game, many of the heroes trade in their personally-customized Gears for LostMagitek with superior capabilities.
Custom Robo is, as the name suggests, all about ace customs.
Sword of the Stars features an ace custom flagship, a jacked-up CnC mission fused with a barrage mission. It costs a fortune, and you can only have one at a time. Oh, and losing one hits your morale harder than having something death star a major colony.
The main characters of Danball Senki all use special LBX, either customized by themselves or done for them by prominent LBX producers (justified as good players are hired to test new products).
Yasha from Asura's Wrath has a jet bike called the Lone Wolf: Corvette, that has the power of the other deities Septentrion flagships, with speed similarly fitting for his fighting style.
The Mech Warrior series has the MechLab, allowing players to heavily customize their Humongous Mecha to their liking. Some loadouts will simply adjust the armor distribution or how much ammo is carried for each weapon, while more extreme loadouts will share nothing in common with the original loadout; such as a player turning a long ranged missile support mech into a closed range brawler loaded with dumbfire rockets. In the singleplayer games, the bosses generally have Ace Custom loadouts on their mechs, and sometimes the Elite Mooks as well.
Anarchy Reigns have a pilotless example in Garuda, a Transforming Mecha based on the Gargoyle robot. However, while Gargoyles normally have plasma arms fit for ranged combat, Garuda possess a pair of drills instead for close-quarters combat to better fit his partner who prefer blasting enemies from afar with his revolver cannon.
Several of the unique weapons in Fallout: New Vegas. Some of the notable ones include Lucky, a long-barreled .357 magnum revolver with gold etching and an ivory grip inlaid with a club symbol, Maria, a customized 9mm pistol decorated with the image of Our Lady of Guadalupe, used by Benny, the Survivalist's Rifle, a Service Rifle chambered for 12.7mm ammo, A Light Shining In Darkness, a .45 pistol with a custom hammer, snakeskin grip and an inscription of a passage from The Bible, used by Joshua Graham, and La Longue Carabine, Corporal Sterling's scoped cowboy repeater rifle.
Fallout 3 has a number, including the Reservist's Rifle, Dave's hunting rifle Ol' Painless, the Blackhawk revolver owned by Agatha's late husband, Sydney's 10mm Ultra SMG, Ronald Laren's Kneecapper sawed-off shotgun, and Smilin' Jack's Terrible Shotgun.
SD Gundam Capsule Fighter does this, allowing players to modify stats to make their units hit harder or move faster. A lot of times, though, it's just to make you die in one hit.
Star Trek Online does this with their starships. From the ship's outside look to the payload to the shields and engines, no two ships are actively alike.
A villainous example of this is General Hakeev's ship, the Khnail, a D'Deridex Romulan warbird. When you first see it, it's a normal D'Deridex. The second time you run into it, it looks like your usual player Ace Custom with a few Borg gear. The third time, you find out that he's outfitted his entire ship, inside and out, with Borg technology that it no longer looks like a D'Deridex, it looks like the Narada. You scrap his ship during this third encounter and when you run into him again during a later mission, he's back to a normal D'Deridex.
A rare non-weapon variant of this trope is present in the semi-realistic flight sim Sky Odyssey. At the end of each mission players are given parts to customize their aircraft. How much you can buy depends on how well you scored in the previous mission. Players can also give their aircraft custom emblems, paint jobs, and names as well.
The admirals of the Valuan Empire in Skies of Arcadia all have Ace Custom versions of the Valuan airships, designed to accommodate their favoured battle tactics. In addition there's the Delphinus, intended to be the royal flagship of the Valuan armada and both an Ace Custom and Super Prototype for the smaller 'Spectre'-class battleships. Naturally, the heroes nick it, ensuring they have the fanciest ride in the game.
The police cruiser that Stich stole when escaping the prison ship.
Yeah, he took the red one...
The e-frames given to Able Squad in Exo Squad were all modified versions of the usual models, testing new features which inevitably turn the tide in the first battle they are used in.
Phaeton's personal Command E-Frame is capable of shrugging off direct hits that would destroy a normal E-Frame. He later has it further customized with medical equipment to help treat his automutation syndrome.
This was the ultimate fate of the Megas XLR unit: though it started as a stolen Glorft Super Prototype, it was SO heavily modified by Coop that he was the only pilot familiar enough with the controls to actually use the thing, as opposed to its intended pilot, Kiva.
Sym-Bionic Titan: Ilana's mech, Corus, is implied to be this since no others like it were ever shown. Lance uses a standard issue Manus armor with a slightly different color scheme; the Combining Mecha aspect probably isn't standard, though, since the King passed him a new model (his old one had been confiscated) before sending him off.
Computer geeks with their computers, going from customizing the desktop to building the machine from scratch.
Gentoo Linux is aimed at people who want to do this to their whole operating system.
Some Imperial Japanese Navy pilots flying Zeros out of Rabaul during WW2 ordered simple armor plate welded in around the interior of the cockpit. They were a minority but towards the end, the only ones left. Perhaps uniquely for the concept of Ace Custom, they performed worse than the basic model... but were more likely to bring their pilots back to base.
They just sacrificed speed and maneuverability for better armor. The stock Zero was very much a Fragile Speedster.
This is a common habit among soldiers of all kinds. The end result often is that a weapon turns out to be better for a purpose other then that for which it was originally designed.
Carlos Hathcock, a US Marine Corps sniper during Vietnam (who literally wore a giant white feather in his bush hat), built a custom bracket to mount a 10X Unertl scope onto an M2 Browning machine gun which he used for several confirmed kills (including one at 2500 yards).
During World War I, the "Red Baron", Manfred von Richthofen, became one of the most famous users of an "ace custom" with his bright red Albatros D.III with a reinforced spar on the aircraft's lower wing.
The Mallard, the locomotive that broke the speed record for steam trains and has held it ever since, incorporated a number of tweaks to its design that made it an improvement over other engines of its class. Its designer, Nigel Gresley, always intended it as the record-breaker.