"This is an interesting one. For the record I view both verses as choose a tier, so... I'll be acting like they're comparable in most stats.
1: The Lion; Yeah agreed. Ignoring that 40K skillwank is PROBABLY better than the Grand Republic skillwank, the Lion sort of dogwalks him.
2/11: Lolnoinformation
3: Fulgrim; As it turns out when one of your main combat styles is trying to be a skillful swordsman who clashes blades and-- oh, right. Lightsabers in Legends are like 27 million degrees celcius. Fulgrim's main style works against him here.
4: Perturabo; Pancreas said it better than I could. Either Perturabo uses his brain, or he doesn't and dies. Considering his track record,
the latter.
5: Jaghatai Khan; same as Fulgrim, except Khan has stamina like a Slayer. Which means Grievous gets mutual killed 99% of the time.
6: Russ; Pancreas said it pretty well. Just remember that Leman can also get pissed off, yell, and hax Grievous into the next century.
7: Rogal Dorn; TTS Dorn has a better shot against Grievous. Let me repeat that: A fan project played for comedy has a better shot against Grievous. Dorn really, REALLY is bad at direct combat among the primarchs. Like, Magnus-tier bad. In defense, Dorn stomps and Grievous never even makes planetfall. In a fight, he is SCREWED.
8: Kurze; yeah Kurze's Precognition is jedi-tier. Not exactly something to write home about. Pancreas said it well.
9: Sanguinius; So you take someone in the top 3 most skilled primarchs and put him against someone who's not even top 3 of his era in Star Wars. Grievous dies.
10: Ferrus Manus; He dies. Hard.
12: Angron; I actually think Pancreas is wrong, and that Angron dies. Yeah Angron is a monster, and yeah 95% of Grievous' tricks Don't work... but running at General Grievous with no plan is just going to get Gorefather chopped into bits and Angron not soon after. This is why Darth Maul never really could fight Grievous in Canon.
13: Guilliman; Okay, fair point... if Guilliman didn't have the death sword and LITERAL Deus Ex Machina on his side. The Emperor's Sword is the deciding factor. Guilliman is not as skilled as Grievous, but if a spark hits Grievous, he is dead as a dodo. But points still stand. This is an incon, not a Grievous dub.
14: Mortarion; So... entirely depends on if this is Daemon Mortarion or not. If it isn't, Pancreas' points stand. If it is, Mortarion existing has a good chance at gibbing Grievous once they get close because Nurgle's #2 that ain't a proper Daemon and whatnot. Might be a mutual kill, but Daemons respawn. Grievous does not.
15: Magnus; Seperating my extensive headcanon for Magnus with how he actually acts... he gets chopped to bits. Like, bottem 3 of the Primarchs skill-wise and he doesn’t use his main gimmick half the time. If he opens a portal to hell, or decides to mindfuck Grievous, he wins. If he doesn’t, he is dead. Because throwing balls at Grievous is a good way for Magnus to get Guilliman'd. As in Grievous runs in and hands his bright red ass to him in melee. Ahzek Ahriman would stomp Grievous, because he actually does wizard shit.
16: Horus: So... pre-heresy, Horus wins. During the Heresy, Horus stomps so hard lmfao. He's Magnus if Magnus didn't have more Character-Induced-Stupidity than an End Times character. He can and very well probably would just cast power word scrunch on Grievous if he sees tech he doesn't recognize, which would be all of it.
17: Lorgar Aurelion; Same as Magnus, doesn’t use his wizard shit in combat competently half the time and is chopped to bits.
18: Vulkan; Oh look the guy who'd probably resist the lightsaber. And is immortal. Grievous would die. Hard.
19: Corvus Corax can go invisible. Grievous stands no chance.
20: Alpharius Omegon: even if both are present they get stomped. They lost to a guy, hard I might add, that I already said Grievous would dogwalk. No hope. Nada. Zilch. Spies are bad in direct combat, and this isn't spy vs spy, it's a fight."