Why does the Emperor call Guilliman his ‘greatest triumph’?

I was rereading the scene from Dark Imperium where Guilliman speaks to the Emperor, and I was curious about something. I know the Emperor calls, Guilliman his pride, his last loyal son, his last tool, his last hope, all at the same time as calling him liar, thief, traitor etc., with the idea being that all of big E’s filters are gone, and he’s no longer hiding his true feelings. Guilliman sees that the person he loved and believed in is gone, and realizes his worth to his father is entirely conditional, predicated upon his value to him as an effective tool.

However, I was wondering what was being referred to when the Emperor calls Guilliman his ‘greatest triumph’. When he was still conscious, Horus was the favorite (Everyone calls him the best at everything but he kinda seems like a mary sue IMO), and I don’t think Robu was particularly close to the top of the trust list (The Lion given DAOT weapons, Corvus told about Chaos, Horus made Warmaster, Vulkan with the talisman of the seven hammers, plan of Magnus sitting on the golden throne, Sanguinius being Sanguinius), so why does the Emperor call Guilliman of all people his greatest triumph?

Is it because Guilliman had the largest and most efficient legion, was the only primarch of all 20 to create a stable and prosperous empire of his own, and never had any major issues (eg. Angron or Kurze)? Is it because he exemplifies the Emperor’s ideals the best, like having the vision to educate his legion in statecraft for purpose after the great crusade? What do you think the Emperor is referring to here?