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or would it be the age of terra

the age of technology (dark age of technology)

age of strife

great crusade

etc. etc.... which event or era had the greatest impact on the imperium

Vandire's Reign of Terror.

That's when the Imperium of 40k came into being. Prior to that it was quite different.

I would say the Horus Heresy was the single most significant event, though you could argue the events that created the Age of Strife would be worse since it killed the Imperium and made the Crusade necessary in the first place (leading the the Heresy!).

I submit that the single most important event was when the Emperor made the deal with the Ruinous Powers (or whatever other unnamed warp entities) for the warp essence that would eventually become a part of the Primarchs.

This single act set in motion a chain of events that resulted in the stagnation of the Imperium and the living death of humanity's 30,000+ year long caretaker.

If the Emperor had instead instituted a policy of education about and zero tolerance eradication of warp taint, the IoM might be a different place. Imagine if everyone knew what to look for and how to handle it. Imagine if the Grey Knights and Ordo Malleus were commonplace from day 1.

Yes. As it removed the Emperor and (most) of the Primarch's, which fethed everything up.