Ahriman is the master of sorcery, his so-called "master" cannot and will not discard him like some petty cultist!

What do you think happened to Horous?

Id have to agree the Huron was, I mean look at everything he accomplished. It was also right under the Imperials noses(until the end of course) and after they still wrecked shop. Tho is we count Angron(Daemon Primarch) as a Chaos Lord aswell, then he is by far the most effective. And his attack post-heresy actually got further then a any of Abaddon's Black Crusades.

Horus was weak minded.

Ahriman cannot even see how his actions are no longer his own. How can he be considered strong minded?

Horus didn't actually trust the dark gods and plus he believed the Emperor would join him to convert humanity to Chaos.

He is the greatest sorcerer! How can he not be strong minded? Mighty Tzeentch will keep him.

I think we need to look at which one has come back from the dead the most.

As much as I want to say Ahriman, money is on Perturabo. By himself; without suffering horrendous of losses I might add, almost single handedly wiped the entire Iron Fists and a huge chunk of the Raven Guard legions out.

I need to brush up on my lore it seems lol