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 a 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. 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."
D'Argo and Aeryn Sun are both pretty much capable of taking out entire armies by themselves.
Even the Technical PacifistDoctor probably killed hundreds or thousands of random Monster of the Week—he's the only person feared by the genocidal Daleks. River Song actually called him this in The Time of Angels: she promised the clerics the equivalent of an army, and she brought 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" killed 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. This is in stark contrast to the original series, where a Dalek could be defeated by a flight of stairs. Maybe they got an update. They did in the original series serial "Remembrance of the Daleks"
And let's not forget River Song, who has apparantly commited 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 the episode 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!
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 will be on my way...
Cyber-Controller: What is the message from the Doctor?
*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 Twelth Cyber Legion, its likely the people of that reality became all too aware of what testing the patience of "The Last Centurion" would lead too, if Rory thought they in any way threatened Amy's safety.
Jack Bauer from 24. Bauer is a One Man Armed Forces and Intelligence Service. Plus a Badass 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."
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.
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).
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.
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 small blaster 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".
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 the episode 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 were being attacked by Satan.
Person of Interest: Using his CIA training, he can take out some considerably large armed groups, and borders on invincible in hand-to-hand combat. He can be overwhelmed with numbers, though.
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.
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.
The Merlin series finale featured Merlin going up against the entire Saxon army at Camlann, in addition to a High Priestess and a dragon. He wins, hands down.
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.
Melinda May in Agents Of Shield has this reputation. From what we've seen so far, especially in the second episode, it's probably justified.
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."