Sanguinius
Sanguinius is the Primarch of the Blood Angels legion. 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.
Biography
Landing on Baal, he was found by the Folk of the Blood, which raised him to manhood. Sanguinius was actually pretty handy at fighting off those mutants that kept attacking them, and was actually a pretty nice guy for 40k. After the Emperor found Baal, Sanguinius wept a single tear of joy because he's manly enough to be in touch with his feelings, and bowed before the Emprah. Joining the Great Crusade, Sanguinius became best buds with Horus and Rogal Dorn, and the friendship with the former is said to have been closer than any other primarch. Which, being 40k, went straight to hell when the Horus Heresy started (The fuck did he actually do during the Heresy, apart from stand in front of the Imperial Palace and troll Bloodthirsters?). Signus Prime that's what.
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 one of 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 (I guess Leman Russ and Magnus the Red were too busy fighting each other to accept), Horus had received the backing of all four Chaos Gods, and so he slew Sanguinius so hard it mind-fucked his entire Legion. Nowadays, all Blood Angels and their successor chapters have to cope with the Black Rage. Succumbing to the Rage leads to the Black Rage, where they think they're Sanguinius reliving the final battle with Horus (see Angry Marines).
Legacy
Though Sanguinius lost to Horus, it is widely 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 widely 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. However, in the Horus Heresy Collected Visions, it details the battle between the Emperor and Horus, and makes it clear that the only reason Big E didn't kill him immediately is because he thought he might be able to save him.
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 Sanguinus' strength but was much quicker to go all out.
In M41 Sanguinius is the Primarch most beloved by the Imperium for his heroic sacrifice (despite what he who shall not be named would say about our Spiritual Liege), and is a hero of the Imperial Cult. Across the Imperium a celebration called the Sanguinala is held in his honor, where adepts wear the iconography of the Blood Angels (a festival dedicated to Leman Russ would have been a better party, but would also have involved more property damage and alcohol related deaths).
It is a well known fact that every female (human, xenos and damonette) in the the grimdark galaxy wants Sanguinius, including even Khorne and Slaanesh. Many Sororitas cry themselves to sleep at the knowledge that they won't be able to hold Sangy's beautiful golden mane, as he attacks their fortress with his mighty power sword.
| The Primarchs of the Space Marine Legions |
|---|
 Loyalist Corvus Corax - Ferrus Manus - Jaghatai Khan Leman Russ - Lion El'Jonson - Roboute Guilliman Rogal Dorn - Sanguinius - Vulkan |
 Traitor Alpharius/Omegon - Angron - Fulgrim Horus - Konrad Curze/Night Haunter - Lorgar Magnus the Red - Mortarion - Perturabo |