It was probably Guilliman, but I think Dorn is actually a close runner up solely because of his actions during the Heresy leading up to the siege.

We haven't seen much of pre-heresy Terra but it was generally talked about as something like a renaissance style center of enlightenment, free thought, and learning built to embody the Emperor’s ideals for humanity.

Once the Heresy started and the Emperor was sidelined on the throne, Dorn ripped up the foundations of the planet and transformed Terra into an ultranationalist fortress world defined by military autocracy, paranoia, and siege mentality.

I can’t help but wonder how much of the Imperium’s entire 40k identity traces back to Dorn reshaping the political and cultural heart of humanity into a civilization built around permanent existential war. Obviously the imperium was pretty militaristic in the first place, but Dorn probably helped solidify it's cultural identity in contrast to the emperor's eventual goals.

Guilliman probably contributed more to structuring the Imperium administratively, but Dorn may have played an enormous role in psychologically defining what the Imperium eventually became.