Geralt of Rivia is currently one of the most popular video game protagonists in the world thanks to the reputation of The Witcher III: Wild Hunt's developer as well as the general likability of the character. He's likely also the most popular monster-slayer in the world right now, something that would make his progenitors proud.
In fact, Geralt is so good at his job that he's running out of monsters and bosses to fight and kill in his own game world. That's why pitting him against other video game bosses in wishful crossover matchups ought to be interesting. This way, fans can truly see just how powerful Geralt can be or perhaps how helpless he is against the respective monsters of other game worlds.
Can Defeat: Xeno'jiiva (Monster Hunter World)
Geralt has already made a crossover to the Monster Hunter World universe and seeing as he was already a veteran bane of monsters, he's very much welcome there. For that matter, it wouldn't be too hard for Geralt to face off against the de facto main boss of the base game which is Xeno'jiva or what appears to be some sort of alien dragon.
Despite the massive size, Xeno'jiiva is actually one of the easiest Elder Dragons in the game to kill. Some fans can even wager that Geralt has faced more difficult opponents. Xeno'jiiva is slow, lacks good coordination, and has plenty of glaring weak spots. Besides, Geralt appears more nimble and tougher than the average monster hunter in Monster Hunter World.
Can't: Senator Armstrong (Metal Gear Rising: Revengeance)
It's pretty clear that most of the video game bosses Geralt can take on are quite limited to fantasy universes. After all, sci-fi enemies are more overpowered and cheat more through some futuristic lore mumbo-jumbo. As such, Geralt would certainly be no match against the certified nanomachine junkie of Metal Gear Rising: Revengeance, Senator Armstrong.
Senator Armstrong is pretty much impervious to any attack Geralt throws at him. Neither silver nor steel can get through those nanomachine-enhanced skin and muscles. Geralt might get a magical deus ex machina from time to time but it's still no match for Armstrong's plot armor.
Can Defeat: Big Daddy (Bioshock)
Bioshock's roaming Big Daddies are actually considered more imposing and threatening than the actual generic final boss of the game. However, menacing as they are, they're still mortals that can easily be killed with enough bullets and superpowers.
That's why they shouldn't pose much of a problem for Geralt. Geralt can easily use his elemental signs in order to make quick work of a Big Daddy. He's even faced some in-game equivalents of these things in his own game world, like golems and other magical constructs or brutes. Though Geralt would likely feel bad after killing one of these.
Can't: Sephiroth (Final Fantasy/Kingdom Hearts)
When it comes to being the better white-haired swordsman, Sephiroth wipes the floor with Geralt, hands down. He's one of the most powerful villains in the Final Fantasy franchise and sometimes in Kingdom Hearts. Sephiroth has proven that he can be one mean antagonist even against other franchise heroes.
Apart from having a much better version of Geralt's elemental magic abilities, Sephiroth can also revive any fallen ally he has. If Sephiroth wants to go all out, he can simply use his Black Materia to rain down a giant planet-sized meteor on top of Geralt. That's way above the witcher's pay grade.
Can Defeat: Psycho Mantis (Metal Gear Solid)
Here's one modern-era villain from the Metal Gear mythos that Geralt can defeat. Psycho Mantis might be a prodigy when it comes to telekinesis and telepathic feats but Geralt has also faced similar threats and defeated them. Outside of his psychic abilities, Psycho Mantis can't really do much.
All Geralt has to do is use his illusion-dispelling Yrden signs or brew the appropriate potion to make himself more resilient to getting mind-controlled. Afterward, Psycho Mantis is just a sitting duck as Geralt can easily locate where he is using his witcher senses.
Can't: Zeus (God of War Series)
He might have been the surrogate father to the closest thing to a goddess in The Witcher 3 but that likely won't save Geralt from the wrath of Zeus. At this point, everyone knows who that is except the God of War games made Zeus even more furious and powerful.
He's actually one of the most difficult bosses in the series. So it's safe to assume that Geralt won't even have a chance against such a being. Zeus can simply lounge in one of his recliners on Mount Olympus while chucking some lightning bolts in Geralt's way from time to time. Geralt will need Ciri's help and even then, both of them still might lose.
Can Defeat: Dracula (Castlevania)
Dracula is the poster boy of the Castlevania video games and he's even more complex of a character in there compared to his other media counterparts. Still, Geralt has proven in the latest expansion for The Witcher 3 that even the strongest of vampires are no match for him.
Geralt notably demonstrated this when he defeated Detlaff, who is pretty much an angstier version of Dracula. Besides, Geralt has tons of silver tools at his disposal from bombs to swords. He's also nearly a century old, and is functionally immortal unless killed, much like Dracula.
Can't: Diablo (Diablo Series)
One of the most common monsters in fiction, the devil or some form of high demon, has eluded Geralt since his game world doesn't allow for such biblical beings. Geralt ought to be thankful in that case, though, since the most popular devil in video games, Diablo, is just too much for him.
This is a villain that just keeps coming back no matter which hero slew him (or her). In many instances, Diablo can even corrupt anyone who fights against him and seeing as Geralt is still essentially a human when it comes to morals and emotions, he's not exactly safe nor would his beginner magic and silver sword do much against such a creature.
Can Defeat: Mike Tyson (Mike Tyson's Punch-Out!!)
Take off Geralt's sword and it's a pretty fair match since Mike Tyson is some sort of genetic freak according to most of his critics. In any case, a fistfight between these two should be a close match. Geralt's not exactly the best at boxing but his reflexes and potions should give him the edge.
Mike Tyson might be considered as one of the hardest video game bosses in his own game but he is still only human. That should be a problem for Geralt of Rivia. Now, with a steel sword in his hand, then he might just give Tyson more reasons to retire again.
Can't: Ornstein And Smough (Dark Souls)
Last but not least in the long line of overpowered video game bosses that Geralt can't defeat would be the gatekeeper duo of Dark Souls, Ornstein and Smough. Even the endlessly resurrecting Chosen Undead nearly gave up with these two.
Geralt with his limited combat capabilities as per Dark Souls' standards, would become nothing more than a training dummy for these two golden knights. Ornstein has better magic and Smough has better strength. Together, they eat Geralt alive and use his precious swords as toothpicks.