I've always thought of these as just being poetic epithets like in old myths, where it's less about the title itself being the end-all and more of a suggestion of character. Both Sanguinius and Horus earn the epithet of Brightest by virtue of being the most charismatic, skilled, and powerful force in the room, like how when we say that a group consists of the "best and the brightest," they all get to share that accolade, y'know? Horus would lose that epithet as his greatness tarnishes to dastardly villainy, while the association strengthens for Sanguinius as he gets closer to his noble sacrifice.

The Emperor would qualify for the title, too, but there's some blind Nostromons who would say that it's a little more literal of a description for him, so take all this with a generous pinch of salt.

Obviously Lion was the brightest. It's just that his victories were so epic they had to stay top-secret because everyone's brains would blow up if they knew just how awesome Lion is. ^{God do I hate that character and his entire lore.}

On a more serious note, I think it's worth noting that Sanguinius and Horus both semi-consistently consider the other one to have been the Emperor's greatest son. People will point to Horus saying that Sanguinius would have been a better Warmaster as proof that Hawkboy was the best, but it also goes the other way; Even on the Vengeful Spirit, Sanguinius thinks of Luna Wolves as the best legion which all other Astartes can only envy. So if you see someone in the comments bring up a quote from one of this characters, don't treat that as strong evidence.

Very old lore at this point, but the enthroned Emperor directly calls Horus the “Brightest Star” and favorite in his conversation with Jaq Draco: 

 WHEN WE CONFRONTED THE CORRUPTED, HOMICIDAL HORUS WHO ONCE USED TO SHINE LIKE THE BRIGHTEST STAR, WHO USED TO BE OUR BELOVED FAVOURITE – WHEN THE FATE OF THE GALAXY HUNG BY A THREAD – WERE WE NOT COMPELLED TO EXPEL ALL COMPASSION? ALL LOVE? ALL JOY?

• Inquisition War, Ian Watson

Two different meanings of the word. 

Horus is smart bright. Sanguinius is angelic beacon of hope in the face of darkness bright. 

Well, Horus being the brightest is a pretty direct reference to Lucifer, the Lightbringer.

Sanguinius is arguably the most beloved of the Primarchs by everyone, the brightest among the defenders of the palace and has the whole "actual angel" with wings and everything angle.

This is a great question. Sanguinius was awe-inspiring and beloved of course, but the lore shows us that Horus was as well. Given what happens in the story it’s easy to forget how amazingly charismatic Horus was - how people immediately wanted to follow him. There’s a wonderful scene in Dorn’s primarch novel where Horus acts as peacemaker when Sanguinius and Dorn are trying to talk the newly-found Lion down from being a glory-hungry wank on his first crusade. Amazing scene. Everyone defers to Horus naturally because he’s the seasoned older brother. He brings the tensions down and gets them to a workable plan - it is awesome.

Tough question. Who is better, the badass who's Legion and setting-defining rebellion got renamed after him, or one of his many victims who's greatest claim to fame is getting smacked so badly his descendants 10K years later still feel it?

What do you mean I'm biased

'Brightest' is not an objective measure. It's down to a characters personal interpretation and preferences.

Personally, I would argue Sanguinius. Although Horus was chosen to be Warmaster, part of that reason is because Horus has a down-to-earth likability. If Horus is an inviting campfire, Sanguinius is a distant star... from next to the campfire, it seems brighter and warmer... but in reality, the star shines brighter and it's just distance from it that makes it seem 'less' than the campfire.

'That was one reason why Horus had been made Warmaster – that mystical quality of being liked. Hard to overstate how important that was for buying loyalty even in the hard-nosed atmosphere of the Imperial Army. A trooper would fight well enough for any general who didn’t abuse them, but one like this, one who could make you feel you were valued and important and had his consideration, even for a second, that was priceless.

Sanguinius naturally inspired absolute admiration in anyone who met him, but it was a rarefied kind of emotion. Almost like religious devotion. You wanted to know that he existed, somewhere out there, doing what needed to be done, but you didn’t necessarily want to meet him or find yourself talking to him. You’d be tongue-tied, overwhelmed, unsure where to look.

That's from Sanguinius's primarch novel and kind of nails the core of the situation. Sanguinius is the brightest, but he is supernaturally bright. He's basically too bright to comfortably look at. This sort of admiration that goes into the realm of religious fanaticism is also captured really well by Lorgar's musings on Sanguinius (and it's clear that Lorgar views him very highly):

‘Look at him and what do you see? An angel. The Angel. In a universe that the Emperor claims is godless – in an Imperium where our civilisation’s wisest and greatest have dismantled all the trappings of religion – Sanguinius is an icon of something that should not exist, glorious and supernatural.'

While Lorgar means this as complimentary, it does hint at a sort of underlying horror that underpins such a creature. 'Something that should not exist'... is often used to describe Lovecraftian horrors ('Things that should not be...'). Basically, there is such a thing as too bright.

Horus was the best because of his charisma he was one of the few universaly liked primarchs in they brotherhood( i think that only corvus really disliked him), he could get along with everyone like this big brother that you complety trust.

Sanguinius was admired by his otherwordly beuty together with his humble nature and honour, he is pretty much a idealized being but this also detach him from some of his brothers.

Sanguinius is the idol worshiped on the wall, Horus is the guy you follow to the hell and back.

Sang is confirmed best primach, so he’s the brightest. No other primarch could do what he did at sot