So I wanted to know which Primarch, if removed from the Traitor cause and added to the Loyalist one, would be most beneficial to the Imperium.

For this discussion I'd make the following rules:

-Horus himself falls and the Heresy happens no matter what, so we can't just have him be a loyal son and prevent the whole thing from happening. All primarchs that aren't the one you pick remain on the side they are on in canon.

-Magnus still breaks the Webway project in an effort to warn the Emperor, causing the war in the Webway and making it so that the Emperor and Custodes can't just go out in full force and ragdoll the traitor forces.

With these parameters set, which Primarch would you pick to remain loyal? My answer would probably be Perturabo. For me it seemed like he carried the Siege. Without his obsessive efforts to best Dorn, I think the traitors would have a much harder time. And if we add him to the loyal cause, he could in turn deal some serious damage to the traitor forces. What I'm a bit shaky on is how the dropsite massacre would play out. With the Iron Warriors still loyal, the Imperium has a better chance at Istvann V, but they would still be outnumbered, so maybe the IW legion itself could potentially also suffer heavy losses and not be able to do much in the rest of the war. Though I doubt Perturabo would himself die on the battlefield.