Also, the most important house can become the one that carries the most critical planets or aspects in a specific composite, making it a focal point.
This. It's no less true of composite charts than of individual birth charts.
But when you ask which house is most powerful, what do you mean? Most powerful for what? It sounds like you're saying, which one would mean this is a good relationship, or which one would be best for romance vs platonic friendship vs anything else. That's not how composite charts work.
A composite chart just describes how the relationship
as an entity functions. It does
not say anything about the dynamics between the individuals involved or how they experience the relationship. For that, use synastry, not composite.
Because a composite chart is about the relationship as an entity, the relationship as an entity has to exist for the chart to be valid. If the people involved don't know each other, or only know each other superficially, the chart isn't valid. There has to be a well established relationship between them for their composite to have any meaning.
If you have a well established relationship with someone, you already know what kind of relationship it is and what kind of flavor it has. So why, then, would you need to ask which house of the composite chart is the most powerful?