1. Cyberdyne Systems
Hailing from the Cameron-verse, Cyberdyne Systems is best known for triggering Judgement Day, creating Terminators, Time Travel, polymimetic alloys. Before this, Cyberdyne was a high end computer and tech giant deeply in bed with the military industrial complex. In a more reflex genre sci-fi setting, Cyberdyne creates robots, drones, aerospace craft, cybernetic body parts, learning AI, and so forth.
Our second entry from the Cameron-verse, Weyland Yutani is a big deal, starting with alien tech and eventually building star ships as seen in the Aliens franchise. Interesting note, when the corp was created in the writers room it was based off of two of the biggest manufacturers at the time, British Leyland and Toyota. These names could obviously not be used for licensing purposes, so Leyland became Weyland, and Cameron had a Japanese neighbor who happened to be named Yutani. This corp is at the very top of the size scale, as they build warships, droids, planetary terraforming operations, and xenobiology as a form of military contracting.
Omni-Consumer Products, of the RoboCop universe is a vertical megacorp that owns everything from sweatshops making cheap clothing for discount retailers to cutting edge cybernetic and military hardware. OCP is based on the Japanese zaibatsu model, where almost everything is 'in house'. OCP factories make their product, and OCP trucks and ships move it, and the employees use OCP insurance, and the OCP bank, and use OCP made phones, cars, and everything else. Weyland Yutani stands out for building things like the Sulaco, OCP stands out for making 80% of the things in the average person's home.
Coming from Repo! the Genetic Opera, GeneCo is a medical megacorp that has massive control over it's local government. After some undisclosed incident, the demand for replacement organs skyrocketed (pollution is believed to be the cause) GeneCo stepped up with finance option surgery, organ replacement, and basically got the majority of the populace hooked on unneeded surgery, highly addictive painkillers, and their repo department is allowed to kill, to keep organ replacement costs down. Toned down to a less weird rock opera vibe, GeneCo is a by the numbers soulless biotech giant with a massive and powerful PR and lobby arm.
5. Tyrell Corp
The manufacturer of replicants from Bladeruuner, Tyrell is in the business of making artificial people, and is considered a contemporary of Weyland Yutani with there being something of a socio-technological rivalry between Tyrell's replicants, and Weyland Yutani's androids. That being said, considering the size and scope of Tyrell's HQ, it is safe to assume that the corp has significant offworld holdings, and their replicants are being under in/sold to offworld mining operations, hazardous ops, the military, and so forth.
6. Zorg Corp
Zorg Corp is a by the books evil and soulless megacorp that does everything from cab lines to military contracting. The area where Zorg jumps the shark is that the corp is completely and totally in the service of Cosmic Evil (tm) and has servicing ancient prophesies as a core purpose.
ENCOM is a telecom and tech company that designs, builds, and maintains virtual universes, best known from the TRON franchise. As a tech megacorp, ENCOM's bread and butter is basic telecom, providing service and access, servers and hardware, and the rest of the mundane sundry side of the tech industry. On the flashier side, they are a media generation industry selling virtual worlds, video games, MMO-VR-RGPS, and being pioneers in sentient programs, and the arcanotech realm of translating matter into energy, energy into matter, as seen in people being sucked into the computer, and programs being projected out into flesh. This is some serious dark arcanotech because it is moving into the realm of replicators, transporters, pokeballs, and holodecks.
Coming from Blomkamp's Elysium Armadyne is a military/space industry megacorp that builds everything from medical equipment to combat robots to massive space stations. Unlike many megacorps, Armadyne has a strong aesthetic and a sense of style, with their high end amenities looking like something the ultra-wealthy would enjoy as opposed to the more common brutalist, or sterile chrome and lens flare approach that others demonstrate. It can be ventured that District 9's MNU could be a subsidiary of Armadyne, or that Armadyne is heavily involved in reverse engineering prawn tech. Megacorps love alien tech.
9. InGen Technologies Intl
The dinosaur makers behind Jurassic Park, InGen is a genetic engineering powerhouse that has the ability to create dinosaurs from recovered DNA, or make them from scratch as 65 million year old DNA is unusable and they are just actually that good at their jobs. Expanded in the franchise to include military applications, multiple theme parks, multiple islands, private military contractors, heavy construction, and so forth. At it's heart, Jurassic Park is a shadowrun against InGen by a rival gene corp that goes horribly wrong, and happens to have dinosaurs. Because Dinosaurs.
10. Soylent Corp
It is easy to ignore that megacorps are real, and there are several of them in the food production industry, the majority of the food produced in the US comes from ConAgra, Perdue, General Mills, and a few others. The Soylent Corp is a similar food megacorp, with the difference being that their crackers are made of unwanted human population. It demontrates both the horror of industrial food production, and the moral bankruptcy of megacorps.
11. Blue Sun Corp
In the Firefly universe, if something exists, at some point, Blue Sun was involved in it's creation. In the 'Verse Blue Sun is more of a fundamental force than any sort of organization with a face, or manifestation.
12. Ilaria Corp
The Illaria Corp owns and operates Artic Biosystems, which does highly illegal and immortal biological research in massive underground artic bases, and is run by a board of immortals who are following their own agendas. Excellent use of shell companies, facades, and keeping the big reveal hidden for most of the series of Helix.
A megacorp from an unlikely source, How I Met Your Mother, Altrucel is a seemingly friendly corp making sports equipment, but also makes a fortune off of clandestine military industrial applications. The corp is big enough that it scooped up the equally large Goliath National Bank and Mega-Corp. Like galaxies, mega corps are cannibalistic and will devour each other at the slightest opportunity, and each conquest makes them even more bloated and powerful.
14. Xanatos Industries
Xanatos, of Gargoyles and Xanatos Gambit fame, a fledgeling megacorp, Xanatos Industries is still doing large flashy things like planting medieval castles on top of skyscrapers, and indulging in LOTS of cryptozoology, fringe and pseudoscience, and mythos like technology.
15. Union Aerospace Company
UAC, making safer worlds through superior firepower, of the Doom franchise, in involved in space operations, military operations, and some serious dark science, with teleportation and accidentally opening portal to hell and unleashing the end of the world.
On the surface, Vault-Tec is the friendly smiling company building bunkers to protect America from thermonuclear war. Underneath, the vaults are all a series of cruel and seemingly pointless forms of social abuse and crimes against humanity, and the data that could be extracted from the test vaults it questionable, and its value is... pointless? What purpose could the data from a vault filled with puppets and one survivor be? Unlike more megacorps, Vault-Tec exists in the past, and there is no context or hint that the corp survived the War of 78.
17. Shinra Electric Power Company
The villains of Final Fantasy VII, Shinra is in the energy business, and the city building business, and the supersoldier business, and after discovering Wutai, Shinra, a company, is in the business of declaring war on sovereign nations, and no one bats an oversized polygonal eye. The company has a legacy of alien technology, ancient race technology, and the ability to tap into the force of life itself... for electricity.
18. Silph Corporation
A relatively benign and low key megacorp, Silph corp maintains a massive monolopy over the Pokemon universe, dominating the medical, consumer goods, and probably everything from banking to maintaining the infrastructure. In the fan theory that Pokemon is set shortly after a devastating war, Silph might have been the largest surviving entity and took it upon itself to do its best to restore society, in a non-violent and non-military fashion.
19. Umbrella Corp
Superficially a pharma corp, Umbrella's logo and vibe, along with its introduction in the first film, gives it the aura and impression of what a real life megacorp would look like. Banking, investment, insurance, medicine, telecom, and then into the realm of the villainous, viral weaponry and military contracting. Add in the massive underground bunkers, destroying cities in science experiments, and the rest, and add zombies.
20. Wayne Enterprises
Lex Luthor's corp
22. Stark Industries
aka Marvel Megacorp, weapons, power armor, flying aircraft carriers, fusion reactors, robotics and androids, AI.
23. Anaheim Electronics
Anaheim acquired large swaths of Zeon defense industries in the Universal Century period of Gundam, giving them an early monopoly on building mobile suits and gundams, and until they got greedy, selling to both sides, and losing their main contracts because of it.
24. Kaiba Corp
The Kaiba Corp is the driving force behind the Yu-Gi-Oh universe, and is the license holder of the in universe Duel Monsters game, including the stadiums it is played in, tournaments, military options, as well as owning massive amounts of property.
25. Central Services
The only business in Terry Gillian's Brazil, Central Services blurs the lines between being a totalitarian dictatorship and being a highly inept bureaucratic corporation.
In the Dune series, there is the Emperor, and the lords of the great houses, the Navigator's guild, and then there is CHOAM. While thoe first two bicker over fuedal power, and the third is a glorified teamsters union, CHOAM is the economy and industry of an untold number of worlds all held under a single directorship.