When calculating military strength, don't forget to factor in the organization of each nation's 'war machine.' Bretonia's nationalized industry and intigrated armed forces command gives it a distinct edge in this conflict. Also, Rheinland's nationalist government has consistantly listed the military as one of the country's top priority, so they receive extra funding for neat new toys. At the same time, however, if you want to get really complicated, you start factoring in the socioeconomic status of the four houses and how their supply chain acts in tandiem with their military forces, which throws the whole balance of power in Sirius into question. Is Liberty the strongest? They certainly have the best equipt ships on standard (excluding Gallia). But, it's still a toss up between who's the strongest, given the extreme battering Bretonia has received at the hands of the Kusari and Gallic forces, respectivly, the recent shakeup in Kusari politics, the imperial defeat of Rheinland and its subsiquent troubles at home, and the fact that Liberty has still not really kicked its military into high gear.
On the subject of Gallia, I left it out of this brainstorming for the most part, due to it not really being part of Sirius. It has beyond a doubt the most heavily equipt army and the largest supply chain of any force active in Discovery. That doesn't mean they are the best, though.
In short, you could put the houses in a list, but why bother? It's much more fun to examine each faction's individual strengths and weaknesses than it is to compare them to some strange standard.
