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.