Most "powerful" house (within RP)

[Drake]

Liberty is the most powerful (non-Gallic) House, hands down. None of the others matches them economically or militarily.

No non-House faction/organization comes even close to matching the economic or military strength of a House, any House. Combined Hispanic forces would perhaps be roughly equal to a small House, but they don't have the numbers or economy to really compete. Corsairs might have had the best fighters at one point in time, but that was 17 years ago (and didn't make any sense then either).

Gallia is vastly powerful, the way it's been written. And it's completely self-sufficient, apparently able to find any and all commodities that it might ever need in however many systems they have. And they've managed to develop more advanced technology than the entirety of the rest of Sirius, despite living in a isolationist vacuum. (Junker spies don't explain it). As a note, seeing as the Council has managed to liberate two systems from the most powerful technological and military force in Sirius, something none of the revolutionary factions elsewhere have managed (Dublin aside, but it was never heavily populated or controlled), this means that the Council is vastly powerful compared to the rest of Sirius. They have to at least equal a moderately powerful House to have accomplished what they have.

Without the GRN bothering them, the Council could probably pose a substantial threat to a Sirius House. 'Substantial' meaning they could possibly invade and take over.

Everything is bigger and better in Gallia.