Let me say that I support Horus as the most likely Primarch, by far, to be elected Warmaster. It just makes sense. His record speaks for himself, his charisma, and the generally strong, positive relationships he shared with many of his brothers.
But the topic said we're playing with hindsight. While Horus may have been the safe bet, we have the fortune of knowing that he will fall (and I assume that while we know we will fall, for the purposes of the discussion we can not prevent his fall, at least so long as we keep the variables the same IE elect Horus warmaster).
So know that Horus, while the most attractive candidate, is doomed to fail (for the purposes of this discussion). I mean, technically, at this point, picking Angron is better since he has the ever so slightest chance of actually succeeding as opposed to our theoretical Horus that is guaranteed to fail, right?
True. But if Horus, the best candidate for selection as Warmaster by far, failed, it doesn't provide much hope for any of the others!