The Cambridge English Dictionary's "English Grammar Today" section suggests that the latter is preferred. It counsels that degree or focusing adverbs usually go mid-sentence / mid-position, as you've done here, and that when there's more than one verb, placement is after the first auxiliary or modal verb.
Others are likely to say --- not incorrectly --- that this is prescriptivist advice, and that a descriptivist analysis of normal usage will suggest that it can go in either position (and possibly even after "expressed") and that neither should properly be called incorrect. I can't disagree, but I think "will be best" will put you on the firmest footing. They are quite comparable in meaning, regardless.