The original O.M.A.C. from The DCU, whose name is an acronym for "One Man Army Corps"; also a Shout-Out to a WWII medal of Honor winner who got the nickname "One Man Army Klein". Note that this is out of necessity; in a future where every country is nuclear and international tensions constantly threaten to boil over, where the risk of war is both ever-present and too great to comprehend, one man must stop conflict before it grows. Troops take too long to assemble and deploy, and a large scale conflict is not an option in the world that's coming.
In O.M.A.C.'s first appearance, practically his first line is "Evacuate this section [of the building]! I'm going to destroy it!"
The Punisher. Most Marvel characters already consider him not right in the head. Just look at the arc in the grittier MAX series where he responded to his family's grave being defiled by killing 68 organized crime members in one night. Also, he is shown to end up in pretty bad shape after many of his self-assigned missions. It's implied he has to recover for several weeks after the ones in which he's most injured. The new movies have him end in pretty bad shape after each assailant sent against him, and in the final showdown in Warzone he responds believably to the shots that, thanks to his Bulletproof Vest, don't manage to penetrate. For more about the movies, see the Film sub-page.
In the MAX series, one police officer mentions that Castle's body count exceeds 2000 confirmed kills.
Batman. Whether it's dozens of Faceless Goons or merely half the Legion of Doom, you can bet that he'll get through them (without killing them, of course; this is Batman). This can often be drawn up to him being Crazy-Prepared (you can totally see Bruce just sitting around going like "Okay, if a President Evil took over and I was surrounded by the Green Berets, how'd I get through them all?"), but he can do it on the fly, too.
Iron Man, who has been described as wearing a fighter jet, although that's an understatement. Even better when he uses his Extremis to control multiple suits simultaneously.
Physical GodsThe Mighty Thor and The Incredible Hercules from Marvel. Both can handle hordes of regular humans fine, but when serious can fight large groups of super-villains and powerful mystical creatures. Thor fought off nearly every super-villain in the original Secret Wars and has fought through the armies of Asgard, the underworld, and Hell several times. Hercules has fought off all of S.H.I.E.L.D. and Attuma's armies.
Then there's Galvatron, main antagonist of the Marvel UK storyline and Megatron from another time, who regularly takes on entire squadrons of Decepticons and Autobots, and usually emerges unscathed (his foes less so). He has singlehandedly torn apart elite squads like the Wreckers and the Dinobots, and in his last battle, the combined forces of Autobots and Decepticons from the past and future are unable to truly stop him, and only a rift in space and time ends him.
On the Autobots' side, there is Optimus Prime, obviously. Omega Supreme, Fortress Maximus and Ultra Magnus also follow this trope.
Nearly every 'leader' Transformer fits this trope to varying degrees.
Grimlock and the Dinobots are basically this turned into a Badass Crew.
Each of the Phase-Sixers (Sixshot, Overlord, Black Shadow and Tarn of the Decepticon Justice Division) was upgraded by Megatron to be even stronger than him. Phase Sixer refers to "Phase Six" of Megatron's plans: namely, wiping out the populations of entire worlds. Something each of the Phase-Sixers is able to do single-handedly.
The Decepticon Justice Division, which includes a character mentioned above, is basically five of these teamed up. The five of them, plus their mode-locked, near-mindless Pet, are able to slaughter the entire 200-strong crew of an alternate Lost Light, including Ultra Magnus and Cyclonus. Doesn't do them much good against Megatron, though - at the climax of "Dying of the Light", he pulls out all his stockpiled contingencies and rips the four remaining members apart.
Captain America (see trope image): No other hero has been the subject of more angry commanders shouting "Kill him, you fools! He's only one man!" Although he wasn't MEANT to be a One-Man Army, just the first of many. Then stuff happened and the secret of the process was lost, leading to any number of character origins that relate to attempts to replicate it.
Conquest from Invincible. He claims he has conquered entire worlds with his bare hands. In fact, this is the standard Viltrumite strategy. They send a single person to each world targeted by their empire. Very rarely is this insufficient to take over the world in question. The former Regent Thragg makes Conquest look like a pushover, having been trained from birth to be the strongest Viltrumite alive.
Obelix in Astérix is a prime example of this, on more than one occasion taking on entire Roman armies single-handed, for fun. Any of the Gauls are capable of this when tanked up on magic potion, though.
Groo the Wanderer would be one of these, if it weren't for his stupidity. He has an unfortunate tendency to forget which side he's on in the middle of a battle—to the point where he has found himself the only living person left on the field, having single-handedly most of both armies involved.
Wolverine has literally fought hundreds of mooks at once. A comprehensive documentation of his combat history would include the phrase "piles of ninjas" more than once. Per page.
His daughter/Opposite-Sex CloneX-23 is no slouch either. She single-handedly destroyed the entire Facility installation where she was bred (OK, her mother Sarah may have capped a couple mooks during her own escape), and has been seen carving up security personnel and soldiers by the dozens. And oh yeah, add extensive assassination, espionage, and spec ops training to her qualifications.