system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] Who is the single most powerful/influential mortal(!) being in WH40K universe?

And what happens once they're gone? By mortal I mean being able to die of old age in shorter span than is life-cycle of a star. Also no ambiguous stuff like the Emperor, you know Emperor-damn well that He's eternal.
gpt
Ok, so lets break it down.

We can rule out any orks, tyranids, necrons, dark eldar and chaos marines (who are arguably not mortal anyway).  They are individually powerful, but the influence of an individual is limited to his Waargh/Hive etc and any planets he attacks.  Small scale stuff...

The Tau have the organisation and cohesion to unite their empire as one, but their empire is still small.

The Eldar are the same as the Tau.  They will all unite for the right cause, and they get bonus points because their foresight magnifies the influence of any actions they do take.  But ultimately, there simply isn't enough of them.

Space marine chapter masters face the same problem as the Tau.  Incredibly powerful and cohesive, but by design, each individual chapter is too small to shape the face of the galaxy at the behest of one mans orders.

So what we have left is the Imperial Guard.  Fractious, disorganised, poorly equipped and under skilled.  But united in purpose, and available in numbers no one but the orks can match.  Utilised all at once, for a single purpose, there is no force in the galaxy that could stop them (well, except the orks).  

So if the circumstances dictated it, who could wield that power?  An inquisitor might wield a portion of it, but would be removed from office if he amassed too much.  Imperial guard officers etc face the same problem.  Which leaves us the High Lords of Terra, and specifically, the Ecclesiarch.  He alone could, in the right circumstances, break rank with the other High Lords without losing his powerbase, and through religious loyalty, still command the uncountable guardsmen in the galaxy.  We'd have another Horus Heresy on our hands, but he'd win...