The Brazilian Slaughter
Arcane
Jaghatai Khan, because space mongols.
Based.
Also Jaghatai is the only Primarch who approached the Heresy sensibly.
Dude gets out of a war against Orks, to see his brothers killing each other and throwing accusations of betrayal like rice on a wedding.
He's the one Primarch who actually joined the Emperor's side not out of pure loyalty (like Dorn, The Lion and Ferrus), nor out of being attacked by the traitors first (Vulkan, Corax, Guilliman), nor for other reasons (Sanguinius, Leman Russ), but because he actually looked at the facts, examined each side's argument, heard their opinions and then took a wise, informed decision. Dude went from being apathetic about the Emperor (to the point the Emperor thought he was a wild card) to being balls-to-the-walls FOR THE EMPEROR because he realized what the real stakes were.
He also actually helped his own team, unlike CERTAIN PRIMARCHS who kept scoring own goals across the Heresy (coff coff Leman Russ coff coff)