1st among equals not best among equals Also all of the Primarchs were tempted by the Chaos Gods, only half were weak enough to turn (One of which was Horus).
Well, let's see, one primarch thought carefully and did his best to spread his fathers actual ideas for the Imperium up until the imperial creed replaced the imperial truth, built a vast ~500 system mini empire that functioned and remains to even M41 the most stable, safest, and actually probably one of the few nice places to live in the Imperium, and realized the errors that had been made by the Emperor in designing the Imperial Crusade armies and tried to correct those as well, as well as being pragmatic enough to understand. Was he the greatest warrior? No. Not by far. Was he the greatest tactician or strategist? No, though in both he was quite near Horus. Was he the most charismatic? Not by far. But he was, of all the primachs, the one with the longest and most important legacy for humanity as a whole. Even if the rest of the imperium is a hell hole, Ultramar is the last burning beacon of humanities better traits.
I really can't see Girlyman as the best or a contender. He was a dick. Pretty much he saw his ideas on how the Imperium should be as superior to his brothers. Brought the Imperium into stagnation with the Codex which he didn't explain properly to his sons as something that is to be used as guidelines not a holy book to be followed to the letter. Then he blackmailed the Iron Hands into accepting the Codex rather than trying to convince them over to his book and only then would he return Ferrus's head to his sons. You know something that is dicky beyond measure of most other Primarchs. He pretty much spat on the Iron Hands and saw them as lesser because of it and then spat on his brother's corpse because he would use his brother's head a leverage... That'd even make Eldrad pause for a second in level of dickery. Also because someone disagreed with his book he accused his own brother of being a traitor when he was the one who stood with the Emperor as shit hit the fan and Girlyman just sat on Macragge shitting himself that Horus would one day come for Ultramar and needed to build up the possible remnant of the Imperium...
Emperor>primarch>marines Guilliman is the best representation of the Imperium. I don't think he was the best overall, but he is the best representation of what humanity should aspire to be. He strove for balance in most things. But, his vision for humanity clouded his judgement when someone like Dorn disagreed with him.
