W
Wrathcaster
In Know No Fear, it details how none of the other primarchs really trust Guilliman because they always wonder what he's going to get out of working with them. He's also too cold and clinical, not charismatic like Horus and his approach to logistics and empire-building probably irks the others, who likely see him as trying to set up his own empire (which he is essentially). He's the most successful primarch, arguably, but only has 4 friends out of all of them: Ferrus, Dorn, Horus, and Sanguinius.I find it fascinating that Gulliman would want Sanguinus to be the new Emperor. Of all the Primarchs, Gulliman was pretty much the only one to build a successful interstellar empire, and he did so long before the emperor found him. Gulliman screams Emperor v2.0 to me, not Sanguinus. I'm curious as to his reasons.
Sanguinius, on the other hand, is pretty much universally respected and admired by all the primarchs. Never in the whole of the HH books do you really see any primarch or anyone else for that matter disparage or insult Sanguinius, because he was just that charismatic, and also possibly the best warrior of them all. Guilliman would know this, he would know that if any other primarch could have been chosen as Warmaster, it would have been Sanguinius, and even Horus believed Sanguinius should be Warmaster.
No other primarch has true rivalries with the Blood Angels or any quarrel with Sanguinius, so he'd be the perfect unifying force should the Emperor fail, if for no other reason than his stature as a figurehead and symbol of greatness.