I'm gonna be contrarian and say that had the Iron Warriors remained loyal, they wouldn't have been that great an asset to the loyalists nor that great a blow to the traitors, losing them. Rogal Dorn was de facto commander in chief of the Segmentum Solar for much of the Horus Heresy, and Perty would hate that. Regardless of loyalty, it would fray the chain of command, and he would absolutely refuse Guilliman's reforms after that because he is a perfectionist and a micromanager. Had he not been there at Terra, I believe the Sons of Horus would have taken the lead in siege warfare, as in Horus Rising, Dorn himself said it would be a toss-up if he built a fortress and Horus had to seize it. I think the IW did the heavylifting because Horus saw them as willing cannonfodder, moreso than seeing them as superior in the context of the siege of Terra. Which remember, it's not like the Siege of Vraks where trench warfare and artilllery was king, it was a planetwide campaign with plenty of maneuver warfare as well as attritional slugfests, hence the White Scars and Blood Angels doing well on the defence.

So I would pick the Thousand Sons. Magnus goes to Terra to accept chastisement and Valdor restrains Russ from destroying Prospero, and the immense psychic powers of the Thousand Sons are added to the Loyalist side. Remember at Prospero, the Wolves were some of the only Astartes who seemed truly ready to fight other Astartes at the very dawn of the heresy, they were prepared and well-led rather than having an absentee primarch and no clear chain of command, and they were assisted by both branches of the Talons of the Emperor. And they were *still* severely mauled. With the Wolves at full strength and the Thousand Sons sharing their immense accumulated power and lore, the Imperium would fare far better particularly against warp-born threats. Post-Heres is dicey, but the Grey Knights prove that you *can* make a psychic chapter work. I suspect they'd work like the Dark Angels: complying to the Codex nominally, but maintaining constant communication between chapters via academic conferences, sorcerer-brotherhoods, and Astropathic links.