Sanguinius
Sanguinius is the Primarch of the Blood Angels chapter. He was notable for the fuckhueg Angel-like wings that mutated out of his back during his childhood on his home planet Baal, which are now emulated on Blood Angel jump packs.
Though not first in favor among the Emprah's primarchs (that position belonged to Horus, pre-heresy), he was first in power. Of the canonical Space Marines, his chapter is the most batshit-loyal to the Emperor. He also breaks Bloodthirster spines over his knee, and is a pretty cool guy who kills aliens and doesn't afraid of anything. That particular Bloodthirster then trolled the Blood Angels by afflicting their geneseed with the Red Thirst and Black Rage. During the Siege of Terra Sanguinus stood alone guarding the Eternity Gate against an entire Legion and Beat all their sorry asses while the Adeptus Custodes fell back to take a break and get their shit together.
He is dead. Oh boy is he ever dead. When the Emperor went with his forces to defeat Horus during the final hours of the Heresy, Sanguinius got sequestered from the greater force and was forced to fight Horus alone. Though Sanguinius was the most powerful of the primarchs both physically and psychically, Horus had received the backing of all four chaos gods, and so he slew Sanguinius so hard it mind-fucked his entire chapter. Nowadays, all Blood Angels have to cope with the Red Thirst. Succumbing to the Thirst leads to the Black Rage (see Angry Marines).
Though Sanguinius lost to Horus, it is agreed upon that the Emperor would not have been able to best Horus were it not for the chink that Sanguinius put in Horus' armor. And it is also agreed that if Horus didn't come to save his Legion's ass that it would have been grinded out of existence by Sanguinius. Lastly it is also agreed that Sanguinius was tired after several days of fighting, if he were totally refreshed, Horus would have had a lot more than just a chink in his armor.
In short, if Horus is Lucifer and The Emprah is God, then Sanguinius is Jesus.
Sanguinius died for your sins...Just like Optimus Prime does on a daily basis.
Sanguinius also bears the distinction of being one of the few Primarchs (alongside Horus) who were capable of lasting a little more than 3 seconds against Angron. Who had Sanguiunus' strength but was much quicker to go all out.