In Macbeth, which phrase in the letter best summarizes Macbeth's relationship with his wife? (Act 1, Scene 5)

I would choose the phrase that precedes the "partnership" descriptor.  Macbeth says that he wrote of the witches prediction because "This have I thought good to deliver thee."

This small phrase is very illuminating.  He is deferring to his wife.  He "thinks" he should make her informed of this news and that she'll be happy.  He doesn't say:  look, I have good  news.  He doesn't say "this is what we are going to do".  He says he thought it "good to deliver thee".  Macbeth is subservient in all aspects - to Duncan, to the witches, and to his wife.  This describes their marriage because, as later scenes will show, he does what she asks of him.  Hence all the trouble!