I personally don't understand why people think anyone else would have been better for Warmaster than Horus. The problem most people have is that they associate Horus as a pure traitor, and he was the GREATEST of primarchs by far. Chosen by the greatest being in the galaxy to be Warmaster. There was a reason for it. The influence of Chaos is too powerful for ANY primarch to avoid. Chaos warps the body and mind. ANY Primarch that had been tainted by Chaos could have been turned into an agent of Chaos.
Again, Chaos changes, manipulates what it touches and there is no cure. Even to think you can fight the manipulation of chaos is ludicris, if you think about how chaos can manipulate the flesh, I think it would be easier for Chaos to manipulate the electronic impulses in the brain or even the brain itself. When thought of this way, I think it is very telling that discussing who wouldn't turn to choas when tainted is very unproductive unless they have Grey Knight invincibility.
One could say that perhaps a better Warmaster would have been one with less influence over the legions. But by playing Devil's advocate, A Warmaster that lacks the will to drive or manipulate the legions to his will would have slowed or halted the Great Crusade. Horus was that driving force that would have been able to drive ALL the legions to his will, however he was also the same force able to create enough strength to devastate the Imperium the way it did during the Heresy.