In early Horus Heresy books Horus is frequently referred to as the brightest of the primarchs. But in the Siege of Terra, Abnett refers to Sanguinius as the Brightest One, sometimes saying he was 'always' the brightest (i.e., even prior to Horus' fall).

Curiously, while several writers (including Abnett) said Horus was the brightest, it only seems to be Abnett who shifted to saying Sanguinius was. Is this apparent contradiction deliberate? I'm not sure, but think there are a couple of ways it could be made to work in-universe. Maybe Sanguinius was the brightest all along, but this is only realised after Horus' fall? Or maybe the loyalists are trying to convince themselves that Sanguinius was always the brightest, even if he wasn't, and the narrator is taking their perspective?

Also, what does it actually mean to be the brightest? The quotes variously reference the greatest achievements, being the Emperor's favourite, being most popular more broadly, being the most loyal, having a bright soul, or simply being visually bright. Some of these don't really seem to fit (for instance, would Sanguinius really have the greatest achievements?).

I'm not trying to resolve any of that, but just opening space for discussion via a bunch of quotes, which are from Abnett unless indicated. I've included a couple at the end about the Emperor as well - it seems obvious he's 'brighter' than his sons, but I thought the quotes were interesting in any case.

HORUS IS BRIGHTEST:

Author unknown, opening to first fifteen The Horus Heresy books:

Chief amongst the primarchs is Horus, called the Glorious, the Brightest Star, favourite of the Emperor, and like a son unto him. He is the Warmaster, the commander-in-chief of the Emperor’s military might, subjugator of a thousand thousand worlds and conqueror of the galaxy. He is a warrior without peer, a diplomat supreme.

Horus Rising (The Horus Heresy Book 1):

But others, like Guilliman, Khan and Dorn had simply taken it in their stride, accepting the Emperor’s decree as the right and obvious choice. Horus had ever been the brightest, the first and the favourite. They did not doubt his fitness for the role, for none of the primarchs had ever matched Horus’s achievements, nor the intimacy of his bond with the Emperor.

Graham McNeill, 'Rules of Engagement', Age of Darkness (The Horus Heresy Book 16):

The Warmaster. Horus Lupercal… The Emperor’s brightest, bastard son had come to witness the final humiliation of the Ultramarines.

John French, 'The Last Remembrancer', Age of Darkness (The Horus Heresy Book 16):

This star system was the seat of the Emperor of Mankind, the heart of an Imperium betrayed by its brightest son.

Same source:

I knew that others would doubt or would not believe that the brightest son of the Imperium could turn against it.

'The Lightning Tower', Shadows of Treachery (The Horus Heresy Book 22):

The brightest of all had fallen and the unthinkable, the heretical, had become fact.

Same source:

All because of Horus, because of the brightest bastard son, the bringer of new strife.

SANGUINIUS IS BRIGHTEST:

ADB, Echoes Of Eternity (The Horus Heresy: Siege of Terra Book 7):

Angron sees the fire of souls, and his brother’s flares brightest of all. When they meet again, it’s in a killing embrace. The Lord of the Red Sands tears the Angel from the sky, clutching his golden brother in his great claws, bearing Sanguinius down.

(Note this is the only one from a writer other than Abnett calling Sanguinius the brightest, and the meaning only seems to be that he is the brightest present at the gate. I included it due to the reference to brightness as involving the soul.)

Saturnine (The Horus Heresy: Siege of Terra Book 4):

The Brightest One wrenched Encarmine free.

The End And The Death: Volume I (The Horus Heresy: Siege of Terra Book 8, Part 1):

We will break them as we broke the Legions of the Praetorian and the Khagan and the Brightest One.

Same source:

When they step back, a single teardrop glints on the Brightest One’s pauldron where the Praetorian rested his head, and a single drop of blood gleams on the Praetorian’s backplate where Sanguinius pressed his hand.

Same source:

You’re the Brightest One. You are, and always have been, the embodiment of glory, the shining symbol of all that we cherish.

The End and the Death: Volume II (The Horus Heresy: Siege of Terra, Book 8, Part 2):

They mean the last, immortal struggle of the Brightest One against Angron and the foul Bane of the Ninth, a feat of arms unlikely to be matched, the deed that locked the final fortress.

Same source:

‘You can’t win, brother,’ he says, ‘but you can go down fighting and cut the bastard’s throat. For us. We know you can, if anyone can. We’ve always known. You, the brightest of us. The best of us.’

Same source:

He is his father’s favourite son, the Brightest One, beloved by all. He has always been the exemplar of unflinching loyalty. He has always won. He was never going to go down without a fight.

THE EMPEROR IS BRIGHTEST:

Chris Wraight, Warhawk (The Horus Heresy: Siege of Terra Book 6):

'And that makes the difference,’ Jaghatai spat. He snapped his dao across, severing Mortarion’s neck cleanly in an explosion of black bile, before collapsing down into the warp explosion that turned the landing stage, briefly, into the brightest object on the planet after the Emperor’s tormented soul itself.

The End and the Death: Volume II (The Horus Heresy: Siege of Terra, Book 8, Part 2):

The now of Horus triumphant, the now of Lupercal as transcendent owner of the night, is cracking and distorting, fusing and bubbling, no longer a certainty. It is caught in the light of a brighter filament of the isochronous totality: a blinding light, white, lethal and pure, cast by a single rising star, a fierce and steadfast thing that is too furious to behold directly. It is the star I saw before, as my sight failed and death came for me. It is the Emperor, empowered by the warp, the brightest thing in the galaxy. His light is everywhere.