Jack Bauer from 24. Bauer is a One Man Armed Forces and Intelligence Service. Plus a grandpa.
Finally lampshaded in Season 6. Jack (alone) storms the terrorist stronghold and kills all the terrorists, and then hangs the Big Bad. When his backup arrives moments later, his partner just looks at the carnage and says "Damn, Jack."
Melinda May has this reputation. It started when she had to fight through a small army of enemies to save a unit by herself in the backstory. Her nickname is "The Cavalry" for that reason: they needed the cavalry to come in and help beat the more numerous foe... and she did. She was a much happier person before that, and she hates the name. We later learn why: The enemy agents were mind-controlled by a child driven insane by her powers and who could will anyone under her thrall to simply fall dead, an ability she was very trigger-happy with. May had to kill the little girl when was on the verge of possessing the SHIELD agents. Now, she does have plenty of standard one-woman army moments, such as the time she escaped from torture and used the knife she'd been stabbed with to kill all her captors, or pretty much any time she's taken down super-powered foes who've previously defeated dozens of redshirt agents. She's pulled off feats more impressive than what she supposedly did in Bahrain so long ago.
Grant Ward also counts. In the first episode, he storms a facility alone.
Bobbi Morse shows off this skill in her first episode, taking out every person who is after Simmons without breaking a sweat.
By the second season, May's training has turned Skye into one of these, capable of single-handedly slaughtering ten HYDRA agents in less than a minute, without even using her powers.
Mike Petersen, period.
In Agent Carter, Peggy Carter counts as one. In the Agent Carter one-shot, she does excellently on a mission intended for 3-5 agents all by herself. In the series, when cornered by her fellow SSR agents at the automat, she knocks them all out with just her fists and a smidge of help from Jarvis. Chief Dooley is astounded that she took out a whole team of trained operatives.
On Almost Human the XRN prototype police android developed a psychopathic personality and went on a killing spree during a presentation. After it killed numerous civilians the police laid siege to the building. After a 36 hour running battle it was finally disabled and all but its head destroyed. It killed 26 police officers and destroys an even greater number of MX police androids. When it gets reactivated in the present it has to use an inferior body but it still manages to easily take down the first police squad to confront it during its rampage. Only John's heroic actions prevent it from killing more police officers.
Lucas Hood, the main character of Banshee. He will tear apart MMA fighters, prison tough guys, and entire gangs. He can take a serious amount of punishment, and dole out even more. Especially notable seeing as he's a fairly average sized guy, and in most cases the only intimidating thing about him is the fact that he's crazy enough to challenge his opponents. There is even an entire episode devoted to fully exploring this part of his character, the strain it places on him, and the distance it makes him feel from others. Also notable for the fact that he gets severely beaten and brutalised while attacking others, unlike more seemingly invulnerable One-Man Armys. Part of his prestige comes from the sheer determination he has to keep going while covered in his own blood.
Chayton Littlestone also qualifies, and is notable for being physically terrifying. He's huge, he's mean, and his voice is the deepest sound you will ever hear. While the protagonist mainly claims this title due to his sheer perseverance and ingenuity of fighting, Chayton is simply a physical force of nature.
Clay can qualify as well, though we still have yet to see him in full action mode against more than one opponent.
Barry: Barry takes on the entire assembled Bolivian, Burmese, and Chechnyan gangsters while he's trying to get Fuches in "berkman/block", methodically killing everyone who stands in his way without getting even a scratch from them in return.
The battlestar Pegasus from both the original and the re-imagined series of Battlestar Galactica is worth a mention. While the crew of the Galactica acknowledges that the best thing to do since the end of the world as they knew it was to guard what's left of humanity and run, the crew of the Pegasus decides that it's a great time to go on the attack.
You would too, if you were commanded by Lloyd "Sea Wolf" Bridges.
In Boardwalk Empire Richard Harrow walks into a brothel occupied by gangsters while carrying a sniper rifle, shotgun and two pistols. He opens fire as soon as he steps through the door and he keeps firing until there is no one left alive on the main floor. He then proceeds upstairs where he kills more gangsters. He kills a dozen men on screen and more off screen. Throughout this he seems to almost never miss. Most of his shots are head shots or center-of-mass shotgun blasts. If we count all the shots he fired, he could have killed up to twenty men without taking a single wound himself.
Lampshaded in the Burn Notice pilot. Michael's new landlord Oleg—apparently himself a former Russian spy—mentions that his agency thought "Michael Westen" was a full black-ops team because "one person cannot make so much problems." He gets a lot of help from his friends in the show itself, but they often work from the shadows to make it appear that Michael is a One-Man Army to intimidate the villain of the week, making it easier to settle matters without a killing spree.
And, from a Russian Black Ops team discussing surrender:
He's Michael Westen, there are only four of us!
In "Friendly Fire", Michael dresses in a black suit with a red shirt and tie, purposely invoking the subconscious idea that he is the Devil. The barrio gangsters he was trying to intimidate are skeptical in the beginning, but by the end of the episode actually believe that they're being attacked by Satan.
Michael invokes this when trying to protect a charity medical clinic from a local gang. Michael could take out the gang out by himself but in a few weeks another gang would move in and start stealing from the clinic. Instead he makes the gangers believe that the doctor running the clinic is a retired Special Forces soldier who could wipe the gang out by himself but is tired of killing. Not willing to take on such a man, the gang agrees to stay away from the clinic and make sure that other gangs stay away as well.
John Casey of Chuck is a Made of IronColonel Badass who can tear through mooks by the dozens without breaking a sweat. His response when confronted by a poisoner who cockily remarks he's about to administer enough poison to kill an army platoon while Casey is restrained to a chair?
Sarah is no slouch herself. When Chuck was captured by spies attempting to extract the Intersect to sell to the highest bidder, she single-handedly tears apart Southeast Asia looking for him and is fearfully dubbed the Giant Blonde She-Male.
For that matter, Chuck himself, when the Intersect is working.
Matt Murdock, AKA Daredevil, is definitely one of these. In the second episode "Cut Man", Matt, after being badly injured, attacks nine Russian gangsters holding a boy hostage. Every one of them gets knocked out and Matt still has enough energy to carry the boy home. He then tops this feat in Season Two, Episode Three "New York's Finest", where, after receiving a beating from the Punisher, Matt knocks him out, and then proceeds to demolish no less than a dozen members of a biker gang, the same one that Lorelei possessed.
Frank Castle, AKA The Punisher, is one as well. He takes out entire gangs on his own, in single encounters. In fact, he was initially mistaken for an entire military assault squad; he killed everyone so quickly and brutally that everyone assumed the damage was done by a team, and there were no survivors to say otherwise. Even when captured by the Kitchen Irish and tortured for hours, he still manages to kill a few members before being saved by Daredevil. When is sent to prison, where he has no weapons whatsoever, he takes on ten other inmates sent to kill him, and still kills nearly all of them, knocking out the rest. At the end of it all, he is covered in the blood of his victims. This is taken even further in his own series, where in one flashback he takes on at least THIRTY Mujahideen soldiers, first with his Heckler & Koch HK416, then his side-arm, then an enemy's AKS-74U then his K-Bar, and finally finished things off with Good Old Fisticuffs and a rock. He even got shot twice and that didn't slow him down.
Jessica Jones's super-strength has allowed her to win bar brawls against entire groups of men and successfully defeat whole groups of people brainwashed into attacking her, despite having no combat training.
Luke Cage has his moments. In the third episode "Who's Gonna Take The Weight", he goes into multiple complexes, sometimes "fighting" as many as ten people at a time, some of them armed with heavy assault rifles and others with knifes and baseball bats. He walks out without a single scratch.
Danny Rand, AKA the Iron Fist, and his girlfriend and partner Colleen Wing are both this. Danny's Iron Fist allows him to take out entire squadrons of men just by slamming it into the ground, and Colleen once takes on an entire street gang of teenagers who've been trained in martial arts all by herself and wins with no more than very minor injuries.
Deadliest Warrior: In the infamous final episode which pitted vampires against zombies, all parties involved agreed that just about 200 zombies against a mere 3 vampires was a fair fight. Unfair, you say? Yeah, for the zombies.
Father Octavian: Doctor Song, you promised me an army. River Song: No, I promised you the equivalent of an army. This is the Doctor.
The Doctor is particularly noteworthy for ending the Time War by initiating an event which destroyed the entire Time Lord race, and almost all of the Daleks (the Dalek fleet alone comprising 10 million ships).
In the new series, the Daleks themselves have become One Alien Armies; the lone Dalek in "Dalek" kills two hundred people in less than an hour, and a Dalek in "Doomsday" boasts (not without reason) that a single Dalek could kill five million Cybermen (though, admittedly, it took them a bit of time to get there).
And let's not forget River Song herself, who has apparently committed so many atrocities, she made a Dalek beg for mercy just by asking it to look up her name, although the fact that history records her as killing the Doctor might have had something to do with it (even if its only because the Doctor faked his death). In "Day of the Moon", she proves herself pretty adept at killing a room full of Silence with a small weapon kept in her purse. What's more, it probably says something about the Doctor that this seems to turn him on a little bit. And God help anyone who gets in their way!
The Doctor: Oh and this is my friend River. Nice hair, clever, has her own gun. And unlike me, she really doesn't mind shooting people. I shouldn't like that... kind of do, a bit. River: Thank you, sweetie!
Rory Williams went up against an entire crew of Cybermen and scared the crap out of them.
Rory: I have a message and a question. A message from the Doctor and a question from me!WHERE! IS! MY! WIFE?! Oh, don't give me those blank looks! The Twelfth Cyber-Legion monitors this entire quadrant! You hear everything! So you tell me what I need to know, you tell me now!. And I'll be on my way... Cyber-Controller:What is the Doctor's message? [the entire fleet behind Rory explodes] Rory: Would you like me to to repeat the question?!
Rory also spent 2000 years in a collapsing history protecting the Pandorica (which held Amy in stasis) and gave "stern warnings" to anyone who attempted to open the box before its time. As we saw from what he later did to the entire Twelfth Cyber Legion, its likely the people of that reality became all too aware of what testing the patience of "The Last Centurion" would lead to, if Rory thought they in any way threatened Amy's safety.
John Crichton pretty much ended a war between the two galactic superpowers single-handed.
D'Argo and Aeryn Sun are both pretty much capable of taking out entire armies by themselves.
In the Fate: The Winx Saga episode "A Fanatic Heart", once she transforms, Bloom is able to tap into enough power to singlehandedly defeat a dozen Burned Ones virtually at the same time, a feat that had been previously established to require multiple fairies working together or an entire battalion of specialists.
Barristan Selmy takes on fifteen Sons of the Harpy and singlehandedly kills all except one of them before succumbing to his wounds.
During "Battle of the Bastards", Jon charges by himself against the Bolton cavalry and racks up an impressive body count during the fight. Not even Barristan Selmy in his last stand against the Sons of the Harpy was able to take down as many of his enemies as Jon did. (Though Jon fought on a giant battlefield of thousands and those he killed weren't always focused on him, making it not as impressive.)
Stannis personally cuts through a score of Lannister soldiers during the Battle of the Blackwater, and later managed to cut his way through a Bolton army.
The IG-11 bounty droid walks up to a base full of thugs and demands they turn over his bounty. He then proceeds to demonstrate that his confidence was completely justified by cutting through them like wheat. He uses Dual Wielding, rotating body parts, and perfect 360-degree vision to shoot every thug with perfect accuracy, while ignoring blaster fire. He only has trouble when they bring out the E-WEB heavy cannon.
And of course the Mandalorian himself. First he fights off half a bar for a bounty, then he disables a sea monster trying to eat his ship, and when IG-11 gets in over his head, he's the one who gets his hands on the E-WEB and uses it to kill the rest of the thugs. He also manages to single-handedly storm and rescue Baby Yoda from a remnant Imperial garrison. That said, the Mandalorian is a more realistic example as he can be overwhelmed by enough enemies; most notably, he was cornered by dozens of rival bounty hunters and would've been killed without the arrival of his fellow Mandalorians.
Boba Fett, for the first time in live-action media, gets to show off how he earned his towering (in-universe and out) reputation, as he uses a gaffi stick to mangle half a dozen stormtroopers, hitting them with such force that their armor shatters. And then, Boba puts his own armor back on. By the end, half a platoon of stormtroopers are frantically retreating from his rampage.
While the Mandalorian has a great deal of difficulty defeating a Phase Three Dark Trooper, Luke Skywalker is able to cut through the remaining Dark Troopers entirely by himself, and hardly broke a sweat doing so.
NCIS gives us Marine Corporal Damon Werth, who is by himself an even match for Team Gibbs thanks to steroid abuse.
Person of Interest: Using his CIA training, John Reese can take out some considerably large armed groups, and borders on invincible in hand-to-hand combat. He can be overwhelmed with numbers, though.
Power Rangers features this often with the monsters, villains, and particularly Sixth Rangers; each of them has enough power to be called equal or close to the other five combined. Zordon was no slouch himself when still humanoid, as the episode "Power Ranger Punks" shows via flashback.
Played for laughs in Red Dwarf in the episode "Stoke Me a Clipper", Ace Rimmer single-handedly wins a two-front skirmish in World War 2. He drops a Luftwaffe plane and kills its crew, wrestles an alligator, picks off a squad of soldiers with nothing but a pistol on his way down and ultimately leaves only one soldier standing. Oh, and he still finds time to surf on the gator and rescue a princess. Even the last soldier has to remark, "what a guy".
Chloe: I was gonna go check it out, but I thought it would be more wise for me to come get my army of one.
Stargate SG-1: The four original members of SG-1 are one man armies by themselves; as a team, they're more like a weapon of galactic destruction. Consider: in Stargate Atlantis Teal'c and Ronon Dex (another One-Man Army) teamed up and slaughtered at least four platoons of Wraith by themselves. Colonel Carter blew up a sun and wiped out a whole Goa'uld fleet by herself. Daniel Jackson single-handedly froze the entire Replicator army with his MIND. And General O'Neill... where do we begin...
O'Neill... lets see: destroyed Anubis' ENTIRE fleet by sitting in a chair and thinking (and for him, this is sometimes hard); pissed off several, if not all, of the evil rulers of the galaxy at least once (some twice) and killed most of them; don't get me started on all the things in this show that have or will explode due to O'Neill's part in making it possible.
The team of the Atlantis Expedition is on its course as well. John Sheppard has racked a respectable body count of both hostile humans and Wraith. At one point the Genii attack Atlantis and Sheppard is the only military officer there at the time, end result: 60+ dead Genii and Sheppard hurt his hand a little. When Ronon meets with old friends of his and they say the tall tales about him while he was a runner, he was denying everything up to the point they say he is told to have killed a hundred Wraith, to which he responds: "That one seems about right." Dr McKay on the other hand has probably the largest body count, though the fewest direct kills: the doomsday machines he fixes and builds have been the cause of destruction for many Wraith hive ships. Teyla, although implied to be a better pure fighter than Sheppard, has the least impressive body count of the team... though that's not saying much. (It's because she was often put on guard duty for Rodney, and her skill was in hand to hand, not shooting.)
And let's not forget the famous quote summing up Sam's time at Atlantis, from Teyla herself "We defeated the Replicators; we thwarted Michael's plans; and the Wraith are in a state of disarray. All of this happened while she was leader of Atlantis."
The original Enterprise destroyed a machine that pulverized whole solar systems.
Technically it was the Constellation, however the Enterprisedid destroy a single-celled organism that literally sucked whole solar systems dry!
The crew of the Enterprise-D destroyed the Borg cube that wasted a whole armada at Wolf-359 by themselves and defeated the Crystalline entity that destroyed a planet.
In this encounter, Enterprise was unable to defeat the Borg cube in combat, but prevailed by exploiting a weakness in the Borg's Hive Mind.
In the first encounter with The Borg, however, the Enterprise made the Borg think twice when her main phasers dealt extensive damage to the Borg cube. About twenty percent of the cube—which itself qualifies as a one ship army—was visibly obliterated with just a few short volleys from Enterprise.
The Defiant. Though it was intentionally designed as a warship.
Sisko (as interpreted by SF Debris): I should have taught my baby to do more than kill.
Subverted by the Valiant, another Defiant-class ship, from the episode of the same name. While it managed to take out some Dominion fighters with ease, things don't go as well when it tries to take on a much bigger, much more heavily armed battleship on it's own. The battle ends with the ship destroyed and all, but three of it's crew killed. Being crewed by a bunch of inexperienced, overstressed cadets certainly didn't help.
Enterprise routinely had its ass handed to it in early episodes, though.
The first appearance of Species 8472 firmly establishes them as an entire species of one-man armies. Three Borg cubes (any one capable of blasting its way through any two major fleets in the Alpha Quadrant) spot one Species 8472 bioship coming out of fluidic space, start their 'Resistance Is Futile' speech...and the bioship shreds all three cubes before they can finish their first sentence.
Pretty much every Starship made by the Federation is a One Ship Armada, a rule of thumb is each ship has three times the shielding, armor and firepower of any other star nation's ship of equal size and class. What really impressive is none of these are design to''be'' for combat (minus the Defiant).
Dean Winchester in Supernatural. His brother Sam may be the smarter one, capable of tracking down monsters more easily and finding a way to kill them, but Dean walked into a large nest of vampires and slaughtered pretty much the lot of them on his own. This was after he survived an entire year of virtually non-stop combat in what was described as "monster heaven." When he bears the Mark of Cain, he ends up on the winning side of many Curb Stomp Battles, but it's Deconstructed as the Mark of Cain had turned him into that season's Villain Protagonist.
Being based on the Sword of Truth books, Legend of the Seeker has Richard (AKA the Seeker) being able to defeat an army with the Sword of Truth. Unfortunately, due to budget constrains, we only ever see him engaging a few soldiers at a time. Zedd could also count, being the last Wizard of the First Order.
Michonne from the The Walking Dead is absolutely lethal with any weapon she's given. In Season 4, she wipes out a small herd of walkers solo with just her katana, and she's shown at numerous points to be perfectly capable of disabling or killing several humans without a single injury to herself. And even more badass? It's revealed in Season 5 that she's self-taught, having acquired her signature katana after the start of the apocalypse and learning its use via the "School of Hard Knocks" method.
Daryl himself counts as well. Whether its being cornered by a huge horde of walkers, blockaded by a motorcycle gang of Saviors, or a one-on-five fight against a bunch of Whisperers, Daryl Dixon doesnt take shit lying down.
In one episode, he joins a group of amateur vampire hunters with the intention of sabotaging their efforts. But when they come upon an entire house full of rather powerful vampires, it's Guillermo who comes to the rescue, single-handedly killing the vampires and saving most of the other hunters.
In the season finale, his employers are about to be executed (for killings he committed), so he goes to their rescue. Just as the execution is about to occur, he goes into action, wiping out dozens of vampires all by himself. Ironically, this happens just after the Vampire Council dismisses the possibility of him being capable of killing even one vampire.
In "Fausta, the Nazi Wonder Woman", she beat up the first three of four mooks so easily that she interrogated the last one and just let him go. And that was the preamble to breaking out of a Nazi interrogation room, overpowering multiple guards at once, and wiping the floor with everyone else in the room - including the Nazi Wonder Woman.
In "Judgement From Outer Space", she rescues Steve Trevor by overpowering two different groups of Nazi soldiers.
In "Going, Going, Gone", Wonder Woman faced off against a Bruce Lee clone. This was soon after the real Bruce Lee's death in 1973 and when that stock character was understood to be a one man army himself. He actually got two hits in...which did nothing to her. And then she crushed him.
"The Starships Are Coming" had the single largest number of bad guys overwhelmed in one scene in the series when she tossed, flipped, punched over and into a trash pile, deflected bullets from, and lassoed eight men. And turned over a car for good measure.
Mason Steele: She must be executed as an example to all those who idolize her. Get her! [Wonder Woman tilts her head with an expression of "Seriously?" mixed with "Bring it on"] Lead Mook: Well, we gotta so something! [Wonder Woman demonstrates why bullying the dragon is such a bad idea]