How about the Cabal that seems to have knowledge that no single entity does?
Not really, they predicted the logical outcomes of an event based on the principles of farseeing. They didn't possess knowledge that no other single entity possessed. Remember, there were numerous warnings from numerous individuals prior to the Heresy about Horus and his rebellion.
So it means that if the Emperor didn't have his massive army to expand the Empire that humanity would be crushed with or without his psychic powers.
Maybe. But what does that prove?
I have a follow up question here also. If he has such powers why would he not recognize the Chaos taint about one year into the heresy. Horus was destroying giant amounts of planets and/or taking them over.
The Emperor didn't deny that the Heresy was going on, nor did he refrain from allowing Dorn (as acting Warmaster) to fight back. The Emperor wasn't involved in the Heresy whatsoever until the twilight of the Siege of Terra, he was too busy fighting the Secret War. Of course he recognised the Chaos taint, if you read Collected Visions
the rhetoric is very much the Emperor versus Chaos rather than the Emperor versus Horus.
Yet the "greatest mind" as said by some, couldn't figure out Horus and his allies were beyond saving until Horus killed someone in front of his face.
Perhaps he wasn't beyond saving.
It might not provide facts, but if you use deduction, the Emperor was at least emotionally weak minded compared to others. Psychic power? Fine. Most powerful, then I disagree, good decision making is what makes a good leader, and being a deity/immensely powerful psyker does not make you the most powerful or the most intelligent.
I didn't realise we were talking about the "most powerful", I was under the impression we were talking about the "most powerful psyker". Regardless, the Emperor was resigned to confronting Horus personally aboard the Vengeful Spirit years before he actually did (see Nemesis
). Given, he hesitated momentarily during the final duel (not only for emotional reasons, but also out of disbelief of how powerful Horus had become and that he was losing) but I don't think you can criticise him for that.
You're implying that the Emperor made bad decisions. Care to elaborate?