I felt like posting this because, after I hit Commander, I ws testing on which weapons and items I could get at my current rank. I found this info on another site and wanted to post it here, as well as requesting it to be stickied so others who hit a new rank aren't confused as I was.
Ensign / Lieutenant = MK I and MK II
Lieutenant Commander = MK III and MK IV
Commander = MK V and MK VI
Captain = MK VII and MK VIII
Admiral = MK IX and MK X