Episode 25 (actually the third episode) of the Kujibiki Unbalance OVA makes several references to the upcoming final battle of the Kujibiki tournament to decide the next student council. The episode cuts off just before we hear the actual rules of the battle.
The Final Battle on each series of the Mazinger trilogy varies depending on if we are discussing the Go Nagai manga, the anime series or the Gosaku Ota manga:
Mazinger Z: Mazinger Z had two battles at the end of the series. At the final of the anime series, the main characters located Dr. Hell's Supervillain Lair at last. Quickly they began to make preparations for the final battle, but Hell used a last scheme to hinder them as he completed his own preparations. Mazinger-Z, Venus A and Boss Borot stormed Hell's Island, supported by the Japanese army, but Dr. Hell sent against them his last Mechanical Beasts. The three Humongous Mecha got severely trashed, but Mazinger-Z could still work. Kouji stormed the base, destroying and blowing up all what he saw. Hell set his Lair to self-destruct and he and Brocken tried to flee on the aerial fortress Guru. However, Mazinger-Z chased them and they faced off for last time on an aerial battle over the ocean. It was subverted, though, since all of it happened in the second-to-last episode, and the narration -and the scenes involving a smug Gorgon- warned the last episode would be NOT a happy day. The events were very different on the Go Nagai manga (Hell attacked first, deploying several dozens of Mechanical Beasts at once to invade Japan. Ashura and Brocken coordinated their squads in the assault, and the Japanese army used Mass-Production Mazingers against Hell. Finally, Kouji and Sayaka fought alone against the Island of Hell, what had transformed into a humongous Humongous Mecha). The Gosaku Ota episodes are similar to the anime, but Gorgon makes his move while Kouji is storming the base. He goads his Warrior Monsters against Mazinger-Z and backstabs Hell when he is distracted before leaving the base. Enraged and dying, Hell pulls a lever. Hell's Island takes off and flies towards the Institute to crash on it. However, Great Mazinger arrives, defeats the Mykene Beasts, fetches a defeated Mazinger-Z and runs away with it. Meanwhile, the battle has altered the course of the island, and it floats upwards, leaving the atmosphere and losing itself in the space...
Great Mazinger: In the anime version, Great Marshall of Darkness sent several Warrior Monsters to attack the Photon Energy Research Institute as Demonika -his Cool Airship- stealthily approached the Fortress of Science. Kouji fought a powerful Warrior Beast but Tetsuya (who was irrationally jealous) refused to sortie to help him. Finally he deployed Great Mazinger, and Great Marshall chose that moment to attack. A Warrior Monster attacked Tetsuya as Demonika started bombarding the Fortress. Tetsuya was easily shot down and cut off reinforcements. Dr. Kabuto then commited a Heroic Sacrifice to save him. After retrieving his corpse, Mazinger-Z, Great Mazinger, Venus A and Diana A regrouped and attacked Demonika together, blowing the Mykene army up to Hell. The manga versions are similar, but there is a very important difference in the Gosaku Ota version: Tetsuya commited Heroic Sacrifice, self-detonating Great Mazinger to destroy Demonika and the Mykene army.
UFO Robo Grendizer: In the anime version, after his daughter got murdered, King Vega decided he had got it: he had wasted military resources trying take over Earth as planet Vega imploded and everyting he had managed was using up their resources, running out of Robeasts, and losing his commanders, his best soldiers and his daughter. So he ordered his troops to destroy their Space Base to hammer in them the idea of they now had no choice but conquering Earth or die, and he launched a full attack with the remnants of his army. Meanwhile, Duke took off in Grendizer and Kouji, Hikaru and Maria on a new starship. Both sides fought in the space, between Earth and Moon. The Vegan army was finally destroyed in the final confrontation, and Duke killed King Vega.Go Nagai manga version had no a definite ending, but Gosaku Ota version did it, and the final was very different from the anime series. Things went From Bad to Worse. Like in The End of the World as We Know It worse.
Code Geass plays with this trope as when Lelouch uses his army as a diversion while he singlehandedly attacks Cornelia in the last episode of season 1. The playing part comes when Lelouch does win but his army's losing the battle. However, upon closer look, he won the battle, but he did fail his objective, both in knowing who killed Marianne and taking Cornelia hostage.
The actual final battle kicked off with a showdown between Suzaku and Kallen, Lelouch and Nunnally of all people, over control of the nuclear death platform Damocles, and ended with Kallen destroying the Lancelot, but having her Guren disabled, and Lelouch victorious, having taken over the world by shedding his principles and wresting control of the nuclear death platform from Nunnaly via Geass. However, (besidesOrange-kun) he has won at the price of losing everyone who loved and respected him. The viewer is left wondering how this is really his victory, especially since the world loathes him for the atrocities he has done in the name of "justice"; then the actualconclusion occurs, showing his tear-jerking triumph over the Crapsack World he was born in.
For Fullmetal Alchemist, the The Promised Day arc, one that began after a large timeskip during which both sides prepared for the crux of the Big Bad's plan to resolve, appears to be the final Arc. It started at Chapter 84, and has since been rolling up ALL the characters into a giant katamari of awesomeness.
The penultimate chapter (107) is even titled "The Final Battle".
Neon Genesis Evangelion actually has two, one against the Angels and another against their "true enemies", so to speak. Shinji confronts the Final Angel, his new best friend forever, Kaworu, within the Eldritch Location of the Womb of Lilith. Afterwards, SEELE launches its final assault upon NERV without any hesitation and causes one of the most tragic final battles ever.
In Naruto, our hero himself fights a noteworthy one against Pain at the end of the Invasion of Pain arc. It results in a clash of ideals, Weapon of Mass Destruction spam and a long drawn out battle.
The real final battle (bar the final battle with Sasuke) seems to be unfolding at present and is fought between the Allied Shinobi Forces (represented by Naruto + Kurama, B + Gyuki, Kakashi, and Gai) and Akatsuki (represented by the Big Bad Duumvirate of Tobi/Obito and Madara Uchiha, plus the Sealed Evil in a Can version of the Ten Tails.
And now all the other shinobi from the Allied Forces have joined the fight.
Princess Tutu ends with a Final Battle that involves lots of dancing, ravens, and the occasional sword. Given the rest of the series it's not a surprise, but it's taken to an epic level.
Subverted somewhat in the manga finale for Chrono Crusade. There are several important battles and victories leading up to Chrono and Aion's fight in the final two volumes—which covers just two consecutive days. However, when the time comes for the final battle, the perspective cuts away right before the first blows are exchanged, and we're never fully given the details of what happened.
Eyeshield 21. Team America vs. Team Japan. They even went into overtime despite the tournament holder's orders. According to the ending, America won.
Any of the Gundam series. From the original all the way to 00 Gundam, each series has concluded with an awesome final battle between the most powerful mobile suits and pilots.
Gundam Wing, like many others, gets away with a little title lampshading. The actual Final Battle takes up the last four episodes of the series, and the episode immediately before it begins is actually called "Signs of the Final Battle".
The final battle of InuYasha takes place inside Naraku when he transformed into a giant, floating spider.
Dragon Ball Z's final battle is Goku and Vegeta vs. Kid Buu in Other World.
Rurouni Kenshin: The Kenshin Gumi along with Aoshi, Misao and Saitou go to Enishi's island to rescue Kaoru. Once there, it's Aoshi, Yahiko, Sanosuke and Saitou vs. Enishi's four henchmen and, of course, Kenshin vs. Enishi. And before that in the Kyoto Arc, we had Kenshin vs. Shishio for the Final battle of that particular arc.
In YuYu Hakusho, the final arc is the Three Kings saga, and the last fight the main characters are involved in is Yusuke vs. Yomi in the Demon World Tournament. Yomi wins but loses the fight after that.
Episode 26 of Cowboy Bebop. The Real Folk Blues, Part 2. The Final Battle against the entire Red Dragon and Vicious.
Oh-so-averted in With Strings Attached, as the anticlimactic final battle lasts about thirty seconds and is depicted entirely in sound effects. The spellcasters in the warehouse are massively outgunned, having completely underestimated their opposition and been tricked by Paul and George to boot. (The real final battle was John and Ringo's struggle to get into the warehouse in the first place. Afterward, well....)
My Little Avengers ends with the Avengers Storming the Castle (literally, as the villains have occupied the royal palace); while Big Mac goes after Loki personally (regaining Thor's powers along the way), the other Avengers seek out their Dark Avenger counterparts (though they make sure to Opponent Switch, so the Dark Avengers can't overpower them, like their previous fight).
Ace Combat The Equestrian War has chapter 19 and the battle in The Valley of Kings. Mobius defeats Echo, Firefly beats Black Star and he tells her the reasons behind Red Cyclone's thinking. Meanwhile, the other ponies fight the griffins off, the Mirage squadron destroys Fortress Intimidation Red Cyclone along with it. But there is still one more opponent left...
Inner Demons has a two-part final battle. The first part occurs when the bearers of the Elements of Harmony and their allies storm into Tartarus in order to confront Queen!Twilight and purify her with the Elements. The second part kicks off when they succeed in doing so, only for the Darkness forced out of Twilight's body to take on a life of its own; circumstances lead to Twilight and Apple Bloom having to face it on their own.
The Azula Trilogy has the Battle of the Earth Kingdom Royal Palace in Soul of Fire, with three notable mini-battles ocurring roughly simultaenously: Zuko and Mai vs. Xang, Katara vs. Ilook, and Aang and Azula vs. Zhan Zheng.
Star Wars had several. A New Hope had the Battle of Yavin, where the rebels blew up the Death Star. It didn't stick, so Return Of The Jedi had the more definitively final Battle of Endor, where they did it again and got Palpatine, too. (According to the Expanded Universe, this didn't kill the Empire either; it plodded on for several years before surrendering at a Peace Conference.) And in the prequel trilogy, the victory against the Trade Federation at the Battle of Naboo actually ends up playing into Sideous' hand.
X-Men 3: The Last Stand and Magneto's attack on the Alcatraz research facility.
Pirates of the Caribbean: The Curse Of The Black Pearl had the battle at Isla de Muerta. At World's End had the battle at Shipwreck Island, with all the pirates of the world against the entire British Armada.
The battle for Zion in The Matrix Revolutions, with the Zion forces battling the Machines in the Real World and Neo fighting Smith for the final time in the Matrix itself.
Hot Fuzz has Angel's return to the village after his fake death. The only living characters that don't end up partaking in the battle are the Sergeants Turner, Mr. Weaver, the London-based characters and a few minor Sandford residents like Mr. Staker.
Man of Steel: Big time with Zod, ends with Superman twisting Zod's neck 180 degrees.
Pacific Rim: The battle of the breach, with Gipsy Danger and Striker Eureka vs Scunner, Raigu, and Slattern.
Almost every action and war movie has one.
Stargate SG-1 episode "Reckoning", part 2. SG-1, the Jaffa, and the Tok'ra vs. the Goa'uld and the Replicators as the fate of the Galaxy hangs in the balance. Originally was intended as the Grand Finale.
The same applies to the battle of Antarctica in the the two-part episode "Lost City", which was also meant to be a Grand Finale.
Star Trek: Deep Space Nine has the final Battle of Cardassia in the ep "What You Leave Behind". Klingons, Romulans, and Federation vs. the Dominion and all their allies. Massive ship wrecking ensues.
Kamen Rider Ryuki is also a separation from the norm since its Final Battle doesn't even involve its protagonist. Instead, it is Kamen Rider Knight who faces off against Kamen Rider Odin. Besides this deviation, the battle is carried out in total sombriety and Knight only 'wins' when the Big Bad destroys Odin in a fit of rage that ultimately does nothing to prevent the fact that Knight had been dealt a fatal blow and dies moments later after using the Rider War's prize to save his girlfriend.
On the Super Sentai side of Toei's live action Toku Cash Cow Franchises, Choujuu Sentai Liveman had a unique final battle: a regular battle with a Monster Of The Week, with a short break where Yuusuke duels The Dragon, Guardnoid Gash. The Big Bad, Bias, dies with Gash in their collapsing lair.
A pretty epic one in Animorphs too, although the main characters don't directly participate in it. (Jake even describes it as the "final battle.")
In The Lord of the Rings, Aragorn leads Gondor, Rohan, and their allies to fight the armies of Mordor at the Black Gate. Aragorn and the other commandersknow full well that they have a very slim chance of actually defeating Sauron's army, and that their actual purpose is to distract Sauron so Frodo can do his job. Once the ring is destroyed, Sauron's will will be broken and his armies will be much easier to defeat.
The Battle of the Five Armies that ends The Hobbit is rather odd, in that the good armies — the Dwarves, Humans, and Elves (as well as the eagles later on) — are Strange Bedfellows who only discover the army of evil — the Orcs and Wargs — as it attacks. (Gandalf's warning does give them a short time to prepare.)
The Scouring of the Shire later in Return of the King (but not in the movie), has Frodo, Sam, Merry and Pippin leading a rebellion of the hobbits to take back the Shire from the evil men under Saruman's command, culminating in a furious battle that drives the men from the Shire.
The Silmarillion is full of this, Nírnaeth Arnoediad (though not final, both sides build up for a confrontation and the good guys gets wiped to never assemble a credible threat against Morgoth again while Beleriand becomes as dark and edgy as something out of the Norse mythology Tolkien so liked. Then it gets worse... finally the Valar sends Eönwe with the armies of Aman to subdue Morgoth in a cataclysmic battle destroying Beleriand. There are several instances of the Valar and Morgoth battling it out before the arrival of The Children of Ilúvatar.
There was actually a previous final battle with the predecessor of Angband, which destroyed Utumno utterly and defeated Morgoth for the first time. This is also the only one where the Valar acted directly and it's said that the landscape of Arda was shaped by this.
Almost every Redwall book culminates in a Final Battle.
The Lion, The Witch and the Wardrobe, Prince Caspian and The Horse and His Boy also ended in full-blown final battles, at least within the context of the book.
The Battle of Hogwarts, in Harry Potter and the Deathly Hallows. The teachers and students of Hogwarts, plus various relatives, friends, etc, vs Voldemort's assembled forces. So that's two small armies of witches and wizards going at it, along with centaurs, elves, giants, werewolves, Dementors and a colony of giant spiders who see the whole thing as a buffet.
The entire Codex Alera series ended with an epic one of these, as the Vord Queen marshaled millions of soldiers to try to crush the survivors at their Last Stand, while the First Lord attempts to find and defeat their leader. Said leader is highly intelligent, possesses great strength and speed AND powerful furycraft.
Most Nasuverse works end with one. Being multi-path stories, there are generally many variations.
Fate/stay night's three scenarios lead to radically different Final Battles:
Fate ends with Shiro and Saber fighting Kotomine and Gilgamesh, respectively.
Unlimited Blade Works has Shiro facing Gilgamesh alone, not to mention the My Name Is Inigo Montoya moment in an earlier battle.
In Heaven's Feel, Shirou, with his Deadly Upgrade faces Black Saber, with help from Rider, whilst Rin fights Dark Sakura. After he defeats (and kills) Saber Alter, he then proceeds to free Sakura from Angra Mainyu's control, before then fighting Kotomine and, depending on which ending you get, either projecting and fires Excalibur (before dying from a Heroic RROD), or being saved by Ilya, who shuts down the Grail and gives Shirou a new body.
Honor Harrington: At All Costs, Book 11 of the main series, was originally going to be the end of the Arc and set the stage for another series a generation later. The battle at the end of the book was designed as the final battle of Honor's series: The Battle of Manticore is the largest battle ever dreamed about, the sheer number of ships being at least an order of magnitude higher than the next largest battle, and the deaths number at over a million for both sides. The original plan was to kill Honor in the final battle — at the height of her greatest victory — and set the stage for her son and daughter to continue the fight 20 years later against the Solarian League. Then author Eric Flint joined the Wages of Sin subseries, forcing the ultimate Big Bad to enact their plan early and giving Weber a way to continue the plot without killing off the main character. What's worse is this battle, despite the horrors, is not decisive. Honor wins and keeps the Star Empire free, but despite the massive fleet destroyed Manticore has no ability to strike back. Her fleet is the only thing protecting the homeworlds, freeing Haven to create a new fleet and try it again. Then Haven finds out that it had been manipulated into war with Manticore. Haven's President, Eloise Pritchart, takes this a bit amiss and makes an unannounced visit to Manticore itself, signs not only a peace treaty but a military alliance with Manticore, and combines the best of the best in three star nations in the Grand Alliance to face down Mesa and the Solarian League. It's going to be awesome.
The first Conqueror book, Wolf of the Plains, climaxed with Temujin (AKA Genghis Khan) leading a Chin-backed alliance of Wolves, Olkhun'ut, and Kerait against the united Tartars. The second book, Lords of the Bow, subverted this trope - the Mongols have laid siege to Yenking and are preparing to attack it, but Kokchu persuades Genghis Khan to accept surrender at the last minute.
The Star Wars Expanded Universe X-Wing series starts out as the Rebel Alliance's attempt to conquer the galactic capital world Coruscant to put the final death blow to the severely weakened Empire. Which ends in a massive (apparently) Final Battle. But it soon becomes obvious that the Imperial administration considered the planet not to be worth defending against a determined attacker and actually planned to lose the battle, while making sure the New Republic would inherit a massive humanitarian disaster.
The first Warrior Cats series ended with a massive battle against BloodClan to determine the fate of the forest. The sequels took away its finalness, but another comes at the end of Omen Of The Stars.
Happens during Jonathan's coronation in Song of the Lioness With Big BadDuke Roger back and worse than ever, Alanna has to get sage advice from Thom's Old Master, defeats Princess Josiane, and goes to face The Dragon and the Big Bad. Jon is busy using the Dominion Jewel to stop a massive earthquake caused by said Big Bad. Meanwhile George has to face Claw, then he, Rispah Coram, Buri, Raoul, Thayet, Myles, and Mistress Cooper have to deal with the combined forces of Claw's rogues and the Tirragen/Eldorne revolutionaries.
The final confrontation at the end of The Immortals. Numerous characters come Back for the Finale for the fight between mortals and Ozorne's forces. Daine has a final showdown with Ozorne, dragons get involved, and it ends up in the Divine Realms as the gods fight the incarnation of Chaos.
Near the end of the second Daughter of the Lioness book, the plans of La Résistance are pushed up when a prison breakout turns into a full-blown riot, revolts spring up on country estates, and fighting begins in the streets of Rajmuat. Ulasim rolls with it and the rebels lead an attack on the Palace. Meanwhile, Kyprioth and the lesser Trickster Archetypes duel with Mithros and the Great Goddess in the skies and Dove rides above the city on a winged horse to inspire the rebels.
In Norse Mythology, it all comes down to Ragnarok, the Final Battle at the End of the World. First there are three years of war in the world of the mortals, followed by three years of endless winter. Then the Sun and the Moon are caught by Skoll and Hati, the wolves that chase them over the sky, which causes the stars to fall down on earth. As a result, massive earthquakes shake the world and collapse the mountains and flood the land. And that's just for starters. Loki's children - the giant wolf Fenrir and the Midgard-Serpent Jörmungandr - escape from their prisons. And in the midst of all this chaos and destruction, Surtr, the King of the fire-giants, arrives from the south with his entire army to attack the gods in Valhalla, while Hrym and his army of common giants arrive with their fleet to join the battle. Soon the frost-giants come also marching from the north. Heimdall blows his horn and the gods attack the giants and monsters, with all the fallen heroes of Valhalla, the Einherjar, pitching in. And then Loki's third child Hel arrives, bringing with her an undead army made of the souls of every single human that has ever died and is not an Einherjar. Odin is killed by Fenrir, who in turn has his head torn in two parts by Odin's son Vidarr. Thor kills Jörmungandr, but dies from his venom. Garm and Tyr kill each other and Freyr is killed by Surtr. Then Loki and Heimdall also kill each other. As the corpses pile up in heaps and the blood gets hip-deep, Surtr decides that defeat is not an option and, being the God of Fire, takes almost everyone with him by blowing himself up in a massive explosion that roasts the entire world.
Apocalyptic Jewish and Christian predictions often included Final Battles. The expectations of such battles often influenced the way actual wars were carried out.
The Left Behind series books Glorious Appearing and Kingdom Come feature both Final Battles from the book of Revelation, with the Final Final Battle before the Great White Throne judgment taking up only a few pages when God just smokes Satan's entire army into ashes in seconds.
The 2008 BIONICLE storyline was marketed like this, even being referred to as such in-universe:
Kopaka: If this is the final battle... let's make it one to remember."
It was, however, all a set-up for a Wham Episode. The 2010 story was probably supposed to be this, but it got severely hampered as the toy line was suddenly ended and the ending was very rushed.
The Battle for Hoover Dam in Fallout: New Vegas is the final showdown between the New California Republic and Caesar's Legion. The Courier is the deciding factor, determining not only potential allies and ultimately whether the NCR or the Legion come out as victors or are both defeated by either Mr. House or The Courier him/herself.
In an added twist, the Battle of Hoover Dam was actually INVOKED. NCR General Lee Oliver wanted a big final battle to defeat the Legion so that his name will go down in history. Most of the NCR, the Legion, House, Civilians, the Courier, and all the other factions know this is a really really really dumb idea (House even refers to this as "Tunnel Vision"). The only reason the battle actually takes place is because General Oliver moved 90% of the NCR troops onto the Dam, wouldn't allow for small skirmishes to wear down the Legion numbers, or battles to take back the large swath of land in the South Eastern Mojave that the Legion claimed. Since the Legion knew this was going on, they take advantage of the General's desire for one of these and pulls off simultaneous raids on the other NCR bases in the region with most of the NCR force at the Dam. This ultimately also deconstructs the concept of the end game final battle.
Warcraft 3 has a final battle where the scourge tries to get to the world tree with ever larger waves of their undead warriors and demon heros. The battle was so epic, that it was implemented in World of Warcraft in the caverns of time.
World of Warcraft have one itself for each expansion: In Classic, it's against the Lich King's Dragon, Kel'Thuzad in his floating fortress, Naxxramas, after beating your way through four wings of different undead forces. In the Burning Crusade, it's against Kil'jaeden, the de facto leader of the Burning Legion, in the middle of him being summoned through the Sunwell. In Wrath of the Lich King, it's against the titular character, on a giant block of ice held between the spires of his fortress, at the highest point in the game world. In Cataclysm, it's against the giant dragon Deathwing the Destroyer, first on his back tearing away his armour plates, then his ruined and completely insane remains in the Maelstrom, a whirlpool so big that it takes a significant part of the world map. In Mists of Pandaria, it is against Garrosh Hellscream, the leader for one of the two player factions, taking place in the Horde capital, Orgrimmar.
The main quest of The Elder Scrolls V: Skyrim concludes with the player facing off against a god-like dragon. The battle takes place on the fields of Sovngarde where the player is assisted by three of the greatest heroes of Nord history.
The pinnacle of your godly powers in Populous is to declare Armageddon, which turns out every person from their house to head for the centre of the map and fight anyone from another faction, until only one side remains.
Asura's Wrath: Episode 22, A Life Well Lived. Basically, the ultimate culmination of the games infamous Serial Escalation, starting with Asura turning into his planet Sized destructor form, that dwarfs Gaea, from being destroyed by a "small" laser from the True Final Boss, Chakravartin, before flying off into space while destroying planets and stars, including a super giant planet as big as a sun by punching through it, and destroying a solar system sized Red Giant, ending with punching through the forehead of Chakravartin, who has literally become so big he dwarfs galaxies to the point of making them look like tiny lights! And that's just the first battle phase!
Mass Effect has the Battle of the Citadel where Shepard and his/her team face off against Saren. Shepard has to get to the Citadel which has been attacked and overun by Saren and Sovereign who are trying to summon the other Reapers from dark space. You manage to fight your way to the council chamber and then you can convince him to shoot himself at which point he's controlled by Sovereign. At the same time, the Alliance is facing off with Sovereign and trying to retake the Citadel.
Mass Effect 2 has the Suicide Mission. This involves flying through the Omega 4 relay (which no ship has ever survived) and then make it to the Collector Base. Once inside the base Shepard and their crew must fight through hordes of Collectors and your crew (assuming there is a crew to rescue by this point). THEN you have to go toe-to-toe with a Human Reaper, decide to either destroy of preserve the Collector Base and then escape again with your squad who may all be dead unless you earnt their loyalty.
Mass Effect 3 has the Battle for Earth. The Victory Fleet all converge on Earth and attack the Reaper forces there to protect the Citadel. Meanwhile, the Normandy manages to skirt around the battle and drop Shepard down on Earth in London, which has been overrun and practically destroyed. You meet up with Anderson who comes up with a plan to get a small force onto the Citadel via a beam. You and your squad must then race through the city to the beam whilst mowing down hordes of Reaper ground troops to reach the beam, only then for Harbinger to show up and start killing left right and centre. You call the Normandy to pick up your injured squadmates and then you're one your own as you race to the beam.
One of these concludes the "Dangerous Days" arc of Sluggy Freelance, with the good guys pulling out every last ally and trump card they've got to take down Hereti Corp.
The final story arc of Checkerboard Nightmare, "In Continuum", culminates in a massive final battle — saddled with an inherent risk that even doing something as serious as an "ultimate battle royale with the Unraveled" would worsen Chex's canonitis. Yeah, long story.
Coga Suro 's entire fifteenth and final chapter is spent on a Final battle, which essentially comprises of Styx covering half of London with a massive dome, underneath which pretty much everyone gets an on- or- off- panel boss fight.
The last two episodes of Beast Machines are this; especially the last one with it featuring a 20 minute Optimus Primal vs. Megatron fight...which Megatron says is just to pass the time until his drones finish repairing the Spark chamber so he can reformat Cybertron.
From Transformers Prime, we have the battle on cybertron between the autobots, predacons and Unicron poseessed megatron for the fate of Primus's rebooted spark. The show technically could do more with the result, but it ends with the end of the decepticons (except to confirm what happened to Shockwave), and the end of the line of primes due to complications protecting The Allspark.
Season one of W.I.T.C.H. features the aptly-named episode "The Final Battle", in which the Guardians team up with Caleb and the Rebel army to storm Phobos' castle. Of course, it turned out to be not so final, as there was a second season.
Avatar: The Last Airbender actually had four final battles at once. The most important was Aang vs. Ozai which both started first and ended last. There was also Zuko and Katara vs. Azula, Suki, Toph, and Sokka vs. Airships, and Iroh and the White Lotus taking back Ba Sing Se.
The fifth season finale of Teen Titans, just before "Things Change", ends with a massive battle between the expanded Teen Titan army and the Brotherhood of Evil.
Likewise, Justice League Unlimited ended with the expanded Justice League unexpectedly falling into an Enemy Mine with the Legion to defend Earth from the armies of a resurrected Darkseid.
Teenage Mutant Ninja Turtles ends season 5 with this. The series comes back as Fast Forward and then Back to the sewers, but the original 2003 series wraps up with all the characters in one huge battle.
Ninjago: Before the show was Un-Canceled, the intended Final Battle was between Lloyd's Golden Dragon form and the Overlord incarnated in his original form: a giant, purple-black wyvern.
In history, there are both final battles and pivotal battles, but it's rather rare to see a pivotal final battle. Most final battles are lopsided affairs, and tend to be short, vicious, and winner-kills-all affairs with little hope of even survival for the defender, while the fights that actually matter rarely end up actually ending the conflict on the spot. There are some good examples of some wars that end only on the last note, such as the Battle of Alesia, which very well could have ended with Julius Caesar dead and his conquests rendered nil, but instead ended with the Gallic rebellion crushed.
Alesia is a good example, Marius' battles against the Cimbrii and Teutons as well, and another Caesarian battle; Pharsalus where he and Pompeii The Great decided who would be master of Rome. Gaius Octavian ('s general) against Marcus Antonius at Actium is a very good example. A later Roman one, Battle of the Milvian Bridge... Rome had many of those.
Also: The Battle of Phillipi was the last hurrah for the Optimate faction. Brutus and Cassius versus Antony and Octavian. This was Antony's Crowning Moment Of Awesome. He managed to beat both Cassius and Brutus, while Octavian's forces were overrun by Brutus's in the first phase of the battle.
Salamis, Hastings, half the battles in the Hundred Year War, virtually every naval battle in the Imjin War. Basically, any battle containing either a national leader or a 'lynchpin' commander.
Dien Bien Phu anyone?
The Battle of Waterloo is a pretty straight example though. Two almost evenly matched armies, with total victory in the war up for grabs.
Although that may be seeing things a little optimistic from the French point of view as large parts of the Allied forces (about one third of the Prussian field army and the entire Russian and Austrian armies, for starters) did not even see action before Waterloo in the 1815 campaign.
Probably more then a bit optimistic. However it was the last battle of the war and dramatic enough to fit the trope.
Arguably, the Battle of Vittorio Veneto in World War I in late October 1918. While there was already movement for peace, matters accelerated because of this battle. Two evenly matched armies (Italians with British/French support v. Austria-Hungary), with the fate of Italy and Eastern Europe to be determined. Midway through the battle, though, the Austro-Hungarian Empire begins to fantastically self-destruct behind them, literally splitting their army in half. In less than five days, the battle ends with Italy completing its 70-year unification, and the Empire fragmenting into several smaller nations. In turn, this led the Germans to revolt, and force the other Empire (which swiftly became a republic) to eventually call for armistice that ends the war.
Leyte Gulf was the last battle in the epic clash between the US Navy and the Imperial Japanese Navy. It was also, depending on how you count such things the biggest naval battle ever. It is also widely believed that it will be the final naval battle ever.
It wasn't quite the very last sea battle of the war, but it was the final major battle.
Apparently the Japanese navy knew they couldn't win, but asked to go on one last mission to fulfill their duty.
They couldn't have flipped the outcome of the war, but they did at least have an outside chance to wreck the Allied amphibious landing. The Pacific War didn't really have a "final decisive battle". Midway was decisive, but the Japanese still had carrier parity for the next year or two of the war. And Guadalcanal wasn't a crushing battle but rather an additional campaign victory achieved by a series of battles that went both ways.
Technically it was the final large scale naval battle. There hasn't been a clash of carrier fleets, battleships, battlecruisers, heavy cruisers, light cruisers, and destroyers sinceWorld War II.
A part of Leyte Gulf, The Battle of Surigao Strait, has the distinction of being the Final Battle ever engaged in between battleships, in the war that saw the end of the battleship as a primary weapons platform, and the cessation of all construction of that class of ship.
While the War of 1812 was already technically over, due to a peace accord having been signed between Great Britain and the United States, the news hadn't reached either the American or British armies at New Orleans by the time the British launched their attack. It was a fitting final battle (at least for the Americans, anyway) as less than 5,000 American soldiers managed to crush a British force twice their size, saving New Orleans and catapulting General Andrew Jackson to national prominence; in fact, had the peace news come in time to prevent the battle, it is likely Jackson would never have become President of the United States.
The Battle of Bosworth Field. It had Richard III and Henry Tudor literally feet away from each other, fighting for the throne of England, and ended with a climactic betrayal by Lord Stanley which resulted in Richard's defeat.