Ladies, enbies, gentlemen, and sanctioned abhumans of the jury... Let me present a case as to what I think is the most interesting choice for this quest.
[X] The Urizen
You may ask: "Why Lorgar? I hate that guy!" and that is precisely the thing. Lorgar is a very hateable character, being in great part the architech of the Horus Heresy, the first Primarch to fall, and so on and so forth - but he is not a bad character, by any means. His fall is perhaps one of the most drawn out and thoughtful of them all. Chaos is indeed a fundamental force of the universe, one Big E was denying its existence. In his own words - "all I ever wanted was the truth".
Now! We may disagree with him. And that's fine! He's a bad guy. An antagonist. But something not many take into account is that Lorgar not only messed the Imperium real bad in the Heresy - he also laid the foundations for the Imperial Cult! He literally penned the Lectio Divinitatus, the holy book around which all religion in the Imperium is organised! So every act of zealotry, every ignorant book burning, every close-minded dogma... It's also Lorgar's fault!
And now, the matter of the Clone. The Primarch's original fall was a thing of circumstance - Colchis itself, Erebus being present, his upbringing, Monarchia, his musings about the divinity of the emperor, his encounters with chaos entities. But this isn't factor here, not for now. Instead, we have a "pure" Lorgar. An orator, a philosopher, a thinker. One that is now steeped in an era of religious fervor, and darkness. There's many paths such a primarch clone would take. They could become a fiery prophet, renewing the imperial faith. They could see the folly of belief, and try to restore the Imperial Truth as it once was - scientific and rationalist. And most importantly, they will have to contend with a universe which is in its vast majority, his original selves' fault. Every act of chaos, and every act of the Imperium - it's all tainted by Lorgar. He is by far the most influential Primarch in Imperial History, even if never at the forefront. Not only that but their unique skillset as an orator or leader will let us by far obtain the greatest following, and open up a lot of opportunities a mere lonely rogue primarch would take decades to achieve.
Clone Lorgar is the flickering candlelight in a sea of darkness. Let your devotion carry you through, to salvation or damnation.
Consider changing your vote to The Urizen, if you find these words compelling.
[X] Man
I'll be brief: I like men.