The best of them was Horus, obviously... It's been said, written, Empi has said it himself etc. The whole idea of the Gods blessing him with their most sacred of Gifts was that he was the Best of All of Them, truly the Emrah's most loved and trusted son - and even he could be brought to see the truth. If you mean Favorite - Angron. Represents the Emperor's ability and expertise in battle, awesome back-story, got the most things done post-heresy than ANY of his brothers... Overall, my favorite. If you mean coolest - probably Konrad Kurze... Coolest story ever, in my opinion. Great concept, a lot of mystery and hush-hush things surrounding his later life and ultimately - death. Compelling writing to be sure.
Wasn't it that they didn't tell the wolves horus had gone traitor because then they'd not even bother with the sons? Then horus contacted Russ and was like "Hey, instead of arresting Magnus go kill him"
Well Magnus tried to contact the Emperor before anyone when he found that Horus was being manipulated by Erebus and had fallen to Chaos. Then Horus lied to Russ that the Emperor didn't want Magnus arrested but dead, sparking the war on Prospero. It's sad how Magnus was so loyal and kept trying to prove to the Emperor that he was on his side until the end when he was forced to change sides to save his sons and city and survive. Of all the traitor legions i respect the Thousand Sons the most.
Lorgar, considering he was the one who saved Angron's life, while also fending off Roboute, simultaneously. He also took on a warhound titan, twice. But apart from that, it was Lorgar after all that managed to convince many of the other word bearers to serve the dark gods, and it was his first captain, Erebus that convinced Horus to turn on the Emperor.
The Glorious Phoenician! Go try to kill or persuade him otherwise if you dare, 1,999999(999999) primarchs have tried it and FAILED. Period. P.S. If somebody would point that 2 primarchs had tried, consider Guilliman'd never been an integral person. Only shining goodness left depicted in the imperial propaganda archives, meh, hate it.
Well none of the traitor primarchs could be called the best- they all failed to fulfill the role they were created for. They are all weak and as for that petulant, vain and pompous fairy boy Fulgrim killing 2 primarchs pffft! He only accomplished this with aid of a demon. Had he been unaided Manus would have kicked the **** out of him. As has been previously mentioned there really isn't a 'best' loyalist primarch just a favourite. As you might have guessed my favourite is Ferrus Manus.