Vulkan, not because of his great personality of warm compassion with every human being, but because he is... well, immortal.
I would have to say either the Lion or Perturabo. When the lion faced off against the Night Lords, he made them feel each hit and every loss. Never mock the lion, for his rage is endless. Perturabo managed to do the most damage to the Imperial fists before and after the heresy. Not to mention that he was very to the point, no useless decorations, no needless celebrations, he simply brought death to all of his opponents.
I read 70+ novels of 40K, but all in german. That makes sometimes hard for me use the right translation for phrases and quotes. Because the german translation is sometimes realy stupid. It get better the last years, but when I start 2003 to read the first books it was awful.
The best primarch. ... thaaat's... wow. That is a loaded question. Not too long ago I would have said Konrad Curze. But with all the character development happening in the Horus Heresy novels, I have to say Angron is the one whose motivations are the most relatable, whose character is the most interesting and sensible both, and the one who... never mind, I just now while writing this realized that Angron is basically Bane from the batman comics. I'm going to go with Mortarion, because he's cool as heck.
Vulkan, Primarch of the Salamanders. "The Night Haunter personally beheaded the Salamanders' Primarch, ripped out his throat with a piece of cutlery, stabbed him through the chest and virtually tore him limb from limb with his own wicked claws. When these attempts failed to kill Vulkan, Curze had him eviscerated, shot at close-range by hundreds of Bolters, put into a ventilation shaft of a starship's engine and even stripped naked and thrown out of an airlock into the airless void of space. But the Night Haunter's efforts proved all for naught. Each time the Night Haunter thought he had successfully murdered his brother Primarch, Vulkan's body would continue to regenerate back to its former vigorous state, further enraging the Night Lords' Primarch. With his unnatural abilities to regenerate revealed to him, Curze attempted to make Vulkan admit that he was no less a monster than himself. To further torment his brother, the Night Haunter had Davinite sorcerer-priests use the fell powers gifted to them by the Ruinous Powers to ensnare Vulkan's mind and run him through a series of illusionary mental trials where he continuously failed at some noble task, resulting in the deaths of innocents. But even this form of sorcerous torture failed to break the resolute Vulkan." ~Wikia & Lexicanum SPOILER ALERT: He escapes by teleporting to Macragge's upper atmosphere, burning the hell up upon re-entry, and basically gets back up and walks away while whistling a jaunty tune. So not only was he immortal physically, he was super resilient to corruption.
None and all of them, the are the best and worst of humanity.... But i do love the quiet genius of Perturbo...
with the help of a greater deamon of slaanesh. and you would not say that if you read the book fulgrim by grahme McNeil
