Let’s start with the easy. Rabbi is an English-language title. The Hebrew version was used only for those who had the authentic Semichah. It is now applied when speaking English to all who claim any form or rabbinic certification.
Rav and Harav (as well as their alternates in spelling) should be used only when referring to those who posess Yore Yore or Yadin Yadin ordination. There is an additional ordination of Rav u Manhig that is used in America differently than it was used in Europe (and Israel) where it was actually a masters degree. Now, many American rabbis have this in lieu of the more traditional Yore Yoreh.
In Germany (and several Central European countries) there was a title of Chaver which implied one who was a scholar but had not [yet] qualified for Semichah.
In places where there are many, many rabbis with various qualifications, it is nice to have a title that can be used to denote a rabbi of eminent status. Hagaon is one such title.
Maran is Aramaic for “our lord” and is also a title that implies more than a mere rabbi, one who is considered as a spiritual father by other rabbis. While it might sound a bit stuffy to an American, it would sound perfect to a Briton, where you have Mr., the Honourable Mr., the Right Honourable Mr., the Reverend Mr., the Right Honorable Reverand, and the Right and Most Reverend….. to apply different levels of their secular and religious nobility.