Which castle can be considered the best?

for the long term, Conflux is definitely the best choice (if there are no restrictions). It allows for quick capture of outposts and secondary locations, easy starting with fake units, and relatively fast flying units (in terms of disrupting the opponent's strategy). For a rush strategy, either Fortress or Stronghold would be suitable. However, I think Castle is the most balanced and frequently played faction, but I don't like its starting units, and its angels are quite expensive. Plus, if the opponent has a few Conservators, they can be a strong counter to your native tier 7 creatures.