Anyplace where there's loads of Empire factions. Just do missions for them, and gravitate towards those with REP++ or REP+++. These missions rank you up faster.
At this point in time, if your primary concern is RANK, forget the credits. Look at REP and choose those which has the most +'s.
If you don't mind cheesing the system, just set up a sidey for planetary landing and like A thrusters and 4 tons of cargo space, go to Quince, Millerport station, and board hop until you get one with a ton of planetary scan missions. Go down, scan stuff, crash sidey into planet for instant teleport back to station, hand in and board switch some more. You can grind out ranks pretty crazy fast. In a couple hours I made from Master to Squire + 51% of Squire rank as well as 80ish mil creds. I had to do a CZ target mission or two to get one of the factions to Cordial so I could get Civial War scan missions.