Sanguinius.
Not only he reformed and redeemed one of the most batshit insane and bloodthirsty chapters in the Imperium - The Blood Angels(who were also known as the "Charnel Feast"),he remained steadfastly loyal to the Emperor and to humanity,despite his ex BFF Horus trying to turn him to Chaos. Not to mention the Red Thirst he had to deal with and to conceal it from others,even though Horus was aware of it and promised to keep it a secret(this was before he became Mega Heretic).
Incredibly humble,charismatic(he got along with all other Primarchs,no small feat),and a monster in melee combat.
Oh yeah, Pre-Sanguinius Blood Angels were abso-fucking-lutely bugfuck nuts ultra violent cannibal berserkers who received cute nicknames like "Revenant Legion" and would turn every planet they assaulted into massive charnel houses where they would engage in massive amounts of murder and canibalism. They weren't even Chaos corrupted and already made the Khornate World Bearers look like prancing nancy fag boys in comparison.
Sanguinius is pretty much WH40k Uncle Ben tho - he's too good for this sinful world so he has to go, and that's why he matters - because he made his sacrifice, he knew he had to make his sacrifice and gladly went to his fate.
I find it ironically sad that he was always so worried about the Red Thirst, because there's some implication that the Red Thirst is not a bug, its a feature. But he was so afraid of his Legion getting the Missing Legion [REDACTED] treatment, he never had the courage to go and ask the Emperor for help with the Red Thirst.
Still, it was funny when Sangyboi realized that he has In-Universe Plot Armor, because
He's Fated To Die Fighting The Warmaster on the Vengeful Spirit, therefore he can literally give zero fucks and go Full YOLO on Chaos because no matter what happens, they can't kill him until them. So he just proceeds to dunk on Titans and shit