I don't find any one of them more important than the others. If I were to concentrate on obtaining only one of them from the other factions which one I would go after would be more of a game by game decision.
Come to think of it, it would be more of a changing decision based on how any particular game was progressing.
Hence: I have to wonder why you ask this question. If you are proposing a change to the game mechanics, please don't.