Darth Marr is the most powerful in the galaxy as of RotHC?

With his humanity long forgotten and his face unseen, Marr's inner thoughts are impossible to determine. But his desire to leave a strong Empire behind him is unquestioned, and his abilities are second to none.

Not second to none in the Empire; just second to none.

Context seems to be Empire-specific, tbh.

No it doesn't.

*shrugs*

Then all the better. 🙂

It isn't referring to combat ability.

What's it referring to? His abilities to leave a strong Empire?

His leadership, administrative capabilities and decision-making I suppose.

The same codex entry points out that Marr doesn't really fight much since the previous war started to drag on and mainly manages the Empire from the Council chamber.