What's the most diplomatic and politically correct way to refer to non-Jews. Both in conversation, and in terms of this site?
I don't know of a problem with "gentiles", though "non-Jews" seems equally appropriate.
I would suggest that the term "non-Jew" is both precise and neutral, and a perfectly acceptable halachic term as well, as halachic works such as the Shulchan Aruch often refer to א"י or אינו יהודי (non Jew).
I think that if people are talking in English, then the words "gentile" or "non-Jew" should be used. However, the terms gentile and non-Jew are likely to be inappropriate if the if the conversation is in more of a "Yeshivish" dialect, because these are both English words. How about Nokhrim (נכרים) or Zarim (זרים)? The latter terms both mean something along the lines of aliens or strangers.