The Ingame Rankings are as followed (based on Personal Arena Rating) :

Grunt = 0 - 1000 Arena Rating
Sergeant = 1000 Arena Rating
Blood Guard = 1200 Arena Rating
Legionnaire = 1400 Arena Rating
Centurion = 1600 Arena Rating
Champion = 1800 Arena rating
General = 2000 Arena Rating

Warlord = Class Leader
Commander = second in command
High Warlord = Guild Master

NOTE : Ranking is purely symbolic and has no value with regards to who makes decisions. The High Warlord, the Commander and the Warlords make the decisions.