Rank 1: Horus Lupercal
The traitor himself, Horus Lupercal, the titular Primarch of the Great Heresy, is considered the best strategist among the Primarchs. Even before his Chaos corruption, Horus was one of the best fighters among the brothers, as Leman Russ himself confirms.
Unlike his brothers, who were elevated to Daemon Prince status by individual Chaos Gods or the “Undivided Chaos,” Horus received the full power of all the gods – albeit only relatively briefly and without becoming a prince. His plan was to ascend to Chaos God and not merely become a prince.
By the way: the entire story surrounding Horus’ betrayal spans more than 50 books. If you don’t know where to start, here you can find the best books to start with Warhammer 40,000.
It might have actually worked: in combat, Horus defeated Sanguinius, the strongest of the loyal Primarchs at the time. He was able to wound the Emperor himself so severely that he could only be kept alive afterward by the Golden Throne.
If Horus had not tried to kill the Emperor without the help of the Chaos forces in his arrogance, he might have been victorious. After his defeat, in a moment of clarity, Horus ultimately asked his father to kill him so as not to be subjected to the Chaos gods. The Emperor honored his request.
Horus was not only the most powerful traitor Primarch but also the most important – even though he is not even directly responsible for the Brother War, but Erebus is. To illustrate how strong Horus really was, there’s even a series where a gigantic Titan is used to fight against the Primarch.
