Thousand Sons for the Blood Angels. Sanguinius kept secrets of his legion from the Emperor to try and save them from being exterminated, and from the Black Rage and Red Thirst they're pretty close to damnation already.
Emperor's Children
Iron Warriors
Night Lords
World Eaters/War Hounds
Death Guard/Dusk Raiders
Thousand Sons
Luna Wolves/Sons of Horus
Word Bearers/Imperial Heralds
Alpha Legion
Thousand Sons for the Blood Angels. Sanguinius kept secrets of his legion from the Emperor to try and save them from being exterminated, and from the Black Rage and Red Thirst they're pretty close to damnation already.
I suppose the Thousand Sons had the most tragic and avoidable fall, and their reward for serving chaos was eternal imprisonment in their armour for most of them.
May the odds be ever in your favour...
Emperor's Children or World Eaters.
Purely because I love the preheresy colour schemes.
Yes, I am aware of how shallow this makes me.
AUT TACE AUT LOQUERE MELIORA SILENTIO
And that's kind of the point, I think. Magnus wasn't just enslaved, he was made an example of. The Gods of Chaos had to make sure that the other traitor legions saw what happens to those who end up serving unwillingly. It encouraged them to fall more... enthusiastically.
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The Thousand Sons had the most tragic fall; they weren't even really traitors. I don't really need to say more though as everyone else here covered it pretty well!
That's a good interpretation, though it can also be surmised that Tzeentch will be Tzeentch and punishes his "followers" as much as he rewards them.
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I voted Thousand Sons for many of the same reason others have stated. Their downfall was so tragic, it's one of the elements that enriches the HH and adds another motivation outside of the personal reasons that caused most of the other primarchs to fall.
As for who to trade places with? I say Blood Angels. In my re-write, Sanguinius sticks with his buddy Horus and devotes himself to Khorne, embracing his space vampire nature.
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I can't ever really see Sanguinius falling to Chaos. It makes about as little sense as Vulkan. Sanguinius is clearly described as one of the nicest, most human and humane of the Primarchs. He really believes in the Emperor's mission to unite mankind and rule the galaxy, and while he isn't afraid to get his hands dirty to do it, he seems like one of the Primarchs who would be most happy to one day retire to discover what living in peace is like. I can't really see him enslaving himself to the Ruinous Powers in order to plunge the galaxy into endless war.
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I think the Heresy books are doing a great job of skewing the good guy/bad guy dichotomy. The good guys aren't all that good (Space Wolves are executioners, White Scars took their time deciding) but more so, the bad guys aren't just straight up evil - Angron was a victim of the nails and his untimely 'rescue' by the Emperor, Magnus was doing what he thought was necessary for mankind, the same for the Alpha Legion, not to mention that Horus was part of a scheme to bring about his downfall and it wasn't just him deciding to turn evil one day.
In any case, I'd save the Thousand Sons as they seemed the least likely to fall out of ask the chaos legions.
Tough call but it would be a swap of iron warriors for imperial fists, however a thousand son swap with blood angels would be nice