HaRav is the most choshuv. In todays world “Reb” just means that he is a breathing male. Look at Misaskims shiva list EVERYONE is a “Reb”. It used to be that one became “Rabbi” with semicha and “Reb” was for the guy in Lakewood learning full time, but hasn’t become a “Rabbi” yet. Of course, in todays world where most of Lakewood just remains in the geder of “Reb”, so they uped everyone else. Lakewood yungerman, one day after chasunah, “Rabbi”. (Does he know how to paken? NO. Does he know maaros? NO. Does he know anything besides his current sugya (that on even that he is not willing to be tested on)? NO. But he learns, so he is Rabbi.
But really the word “Rabbi” today, could be reform, conservative, or Lakewood yungerman. Rav is usually a posek of sort, and HaRav is Rosh Yeshiva grade.