So, for those of us who prefer our primarchs with a little less heresy in their blood, here's the vote.
I'm nailing my colours to the mast from the off. Corax. Dude has mega cool lightning claws and whip for weapons. He has mad ninja skills. Isn't a glory hog like the rest, yet could beat all the loyalists in a fight. Has super smarts that let him figure how to upgrade the astartes creation process. Isn't emotionally stunted like the rest of them. Oh and he can turn INVISIBLE.