It's taken from a mix of people from the original legions-loyalist and loyalists from the traitor legions-they have no 'Primarch' persay.
Sanguinius... He is possibly the most powerful of all the primarchs (besides Horus of course). During the battle of Terra he defended the Imperial Palace cutting through masses of Traitors and sent one of the most powerful Bloodthirsters in the universe back to the warp Bane style in an apocalyptic rematch. He literally grabbed the Bloodthirster and slammed him over his knee, breaking his back, and through his ass back out Eternity Gate. He also Broke off a plate of Horuse's armor in which the Emperor exploited strike a killing blow on him. Horus did say that Sanguinius should have held the title of "Warmaster"
Actually the breaking off a bit of armour is purely fanon, with a couple of nods in the lore along the lines of"A few people think that..." etc.
Not to mention that the Emperor killed Horus with Mind Bullets, so it's not like armor was a factor in the first place.
Wat. The strongest Primarchs were Magnus, Sanguinius, Vulkan, and the Night Haunter. Dorn and Horus certainly aren't on that list.
Sanguinius, He can survive in a radioactive environment with no protection. Wrestles/Stares down giant rad-scorpions in his spare time growing up. Managed to end war on his home and killed a lot of mutants doing it. He was not a dick to the emperor when they finally meet up unlike his brother Leman Russ who'd rather try and out eat, out drink and beat up his dad than accept the fact he is the god damn emperor and his real dad. His 2 crowning achievements were to break the back of the bloodthirster who broke his legs in an earlier event (as a result of this, creating the red thrist/black rage within his chapter) in mid-air granting him his sweet sweet vengeance and he stood up to a fully corrupted Horus and knowing he would lose the fight, martyred himself in order for the emperor realise his favoured son (Horus) was a lost cause and killed him with mind bullets. Its either him or Konrad Curze who is essentially the "Goddamn Batman of the Grimdark Future."
Sorry have to ask out of curiosity, How was Konrad Curze\Night Haunter one of the strongest in your opinion? I know as I mentioned in my post his is the "Goddamn Batman of the Grimdark Future" but I've never seen Konrad as strong or as powerful as say, Magnus who's psychic capability is unquestionably the most powerful amongst the Primarchs or Vulkan who's been revealed as a regenerating immortal like Wolverine or Deadpool. (sorry to those who have not read the book "Vulkan Lives" for spoilers).
Because Konrad is the single Primarch to have the biggest list of other Primarchs he kicked the shit out of in the Horus Heresy. He certainly isn't the strongest, but he has indeed bested a large amount of them.
