Nearly any character being played by Steven Seagal, anyone? I don't think I even need to specify which film, do I? However, one notable example would definitely be On Deadly Ground (1994).
Ogami Itto from the film Lone Wolf and Cub: White Heaven in Hell, based on the manga of the same name, holds the record for the highest on-screen body count by a single character in a single film at 150!
The newest starship Enterprise from the 2009 Star Trek takes the concept of One Starship Armada to ridiculous extremes. Nero's ship the Narada wrecks an entire squadron of Klingons, a Starfleet armada, and finds spare time to blow up implode a planet. Guess which starship defeats it all by itself?? Seriously, it's as if any ship with the name Enterprise is made of badassinum alloy.
To be fair, there was the teensy matter of the black hole weakening the Narada...
Precisely, the other ships were destroyed before even knowing what they were up against. Enterprise just took out the weapons fire aimed at a certain something also from the future which took the Narada out as above.
Also, Nero deliberately lets it live so Spock can watch Vulcan implode. If he hadn't recognized the ship as the Enterprise, he would have just blown it up, too.
The better example is Spock's single ship vs. the Narada at the end. And at that point, Nero wasn't pulling any punches.
Nero: I want Spock dead now!
Really this trope belongs to the Narada if anything. See above.
In the words of Will Riker, fate protects fools, little children, and ships named Enterprise.
A much, much more straight and literal example comes in the form of John Harrison from Star Trek Into Darkness. The man is unbelievably badass: He takes on a whole squadron of Klingons single-handedly, doesn't even so much as budge when Kirk is attempting to beat the crap out of him, and survives his ship crashing head-first into San Francisco no worse for wear. When you get right down to it, it makes perfect sense considering he's Khan Noonien Singh. The title of One-Man Army is not only deserved, but practically lampshaded all throughout the film.
From the same movie, the Vengeance is designed to be the most advanced and powerful warship in known space. Given that it's referred to as Dreadnought-class, and the tendency of classes in this 'verse to be named after the lead ship, it's safe to assume that there is at least one other ship of that class.
So far, Rambo has killed over 250 people, including background information given in First Blood. He does get a lot of help in Rambo III, and in Rambo: First Blood Part II, he starts out as a commando on a mission to shoot photographs, not the enemy, but he is abandoned by the Army once he starts killing Communists left and right.
He doesn't do that bad in the first movie, either.
Mister. Smith. Confirmed kills: 141. Some with carrots. It gets even more awesome if you consider that he didn't even needed explosives or machine guns, only carrots, handguns and his impropable aiming skills. Considering we don't know anything about his past for sure, Smith might have racked up considerably more kills. Oh, and he was the unabomber.
In Commando, Matrix kills 81 baddies. Including one incident of failed diplomacy, a guy whose neck he broke in public without anyone noticing, and the funny guy he said he'd kill last. He lied.
This, incidentally, makes the eponymous alien hunter in Predator that much more badass. Imagine Arnold concentrating his entire One Man Army power... on one guy. That's basically the third act of Predator.
Cleric John Preston has 118 confirmed kills in the movie. His record lies at 53 people in a single fight. And we don't know how many people he killed before the movie began.
Any and all Jedi tend to the One Man—or being—Army trope, though they really only achieve it if they're main characters—otherwise they tend to die.
Man on Fire: A kidnapping ring, including corrupt Mexican police, run rough-shod over the citizens of Mexico City and defy all attempts to stop them. Thus, they are completely unprepared when John Creasy unleashes a One Man Curb-Stomp Battle on them and kills dozens of them in short order.
Somewhat subverted in this movie, though. John Creasy never attacks an armed group head-on and out in the open. Instead, he ambushes his prey or infiltrates their locations, then unleashes hell.
While he often has friends or an army on his side, Aragorn certainly kills enough Orcs in battle to qualify, and was very much this trope for part of the battle of Amon Hen in the first film.
Aragorn has killed 60 enemies on-screen, but this also includes some Elite Mooks, and he certainly killed much more enemies during off-screen fighting scenes. Just remember the many war sequences. Gimli and Legolas aren't so bad either, as both claim to have killed each like 40 Uruks during the battle at Helms Deep.
Tik-Tok from Return to Oz is a literal example of this trope, even calling himself OZ's "army". This looks patently ridiculous at first glance, with him appearing to be a clumsy copper boiler with a head, two spindly arms and thick legs that make him slower than a glacier... and then you see him single-handedly wipe the floor with a large pack of wheelers who are pure terror until this point in the story. Then as the rest of the wheelers flee, he grabs one in a chokehold and mercilessly interrogates him.
Half the reason Dom Cobb hires Eames for his team is because Eames is an expert forger who can take any form he wants in the dreamworlds. The other half is because Eames is an absolute badass who can hold off most of an army of militarized subconscious projections.
The Punisher, as mentioned in the comics section, perfectly fits this trope, and in the movies, this is no different. Special mention must go to Punisher: War Zone, the third Punisher movie. Frank Castle kills over 70 baddies, using various guns, grenades, chairs, knifes, a metal pipe and his fists. On one occasion, he kills a bad guy with a single punch, literally punching straight through his face.
Evelyn Salt, from the film Salt. A one-woman special forces unit, she is able to single-handedly take out squads of specially-trained security agents and other operatives hot on her tail, improvise a cannon of sorts from a fire extinguisher, cleaning chemicals and the leg of a table, jump off bridges and land on the roofs of trucks unscathed, and fashion darts from freshly-milked spider venom - amongst other things.
In Mulan, the eponymous character singlehandedly defeats a majority of the Hun army and then, upon discovering that a few still survived, got their leader killed. With fireworks. As The Nostalgia Chick put it, Mulan is "the only Disney Princess with a body count. In the thousands."
Each of The Avengers is one of these, but special mention goes to The Hulk, who is all but explicitly called one when Tony Stark and Loki are facing off.
Loki: I have an army.
Stark: We have a Hulk.
Jet Li in many of his movies. He doesn't even need weapons, most of the time. Doubled in The One with two Jet Lis, each of whom is fully capable of taking on an army and winning thanks to the in-universe Conservation of Ninjutsu. Naturally, the final battle involves a good-vs-evil face-off between the two. Face with such awesomeness, even Jason Statham can't do much but stand back and watch.
Jackie Chan also usually plays one of these as well. It's the reason people are hyped up about The Forbidden Kingdom, where he squares off against Jet Li. Also like Li, he has also played in a movie where there's two of him, though, true to his more wholesome image, it's a lot wackier.
Foes fallen from Zatoichi's sword cane are impossible to count. For comparison in Zatoichi. Darkness Is His Ally (1989), Ichi kills 103 of them.
John Carter from...well... John Carter. He takes down an entire army by himself and the bodies have to be pulled off of him.
In The World's EndAndrew turns out to be surprisinglyBadass, felling waves of Blanks at one point without stopping for breath.