Who currently holds the most power within the Imperium of Mankind?

In lieu of the Emperor, whom is currently perfecting his impression of a corpse, who’s orders within the Imperium are seen as absolute, and cannot be questioned or denied, if any?

Basically, the character/group that pulls rank on absolutely everyone within the Imperium, as of the latest setting.

(Sorry if it’s a question that’s asked a lot, I tried looking for other people’s posts on the matter, but they were all a year older or more, and while that may mean that the hierarchy has simply not changed since then, as a beginner I just thought I’d ask here to dot my i’s.)

EDIT: Is there a reason questions on reddit seem to be prone to downvotes?