Depends what you like, I stopped grinding rank recently, though I must say it wasn't that much of a grind, I used Deciat area mainly running taxi and boom data missions, all the one ways that give ++ or +++ rep. I'm still trying to combine my activities with Fed rank increases, and as such I was recently finding it easy to get a lot of boom data missions in the Sol area, but maybe you don't have that permit yet? Anyway I got tired of delivering stuff and decided to work on my combat ranking, so I'm trying to do that at the moment, while incidentally grinding fed rank. Just last night I had a lot of luck with 'destroy X faction ships', flipping between LHS 411 and Ross 860, both of which have fed factions warring vs independent factions right now (as of last night, all could have changed now, but I hope not, cos I still need 11 more ships to complete the missions I picked up). So I'd say Boom if you just want to get as much rep as possible, and if you like combat, any system where your chosen main faction is warring with an independent. I made a lot of rep in CZs last night and also selling the combat bonds after. The map can't really help you find these systems though, since to show up on the map, the controlling faction must be in that 'state' whereas in reality, many systems have wars going on but not with the controlling faction. The easiest way to happen across them in the area you want is to do boom data missions and keep your eye open on the Nav screen for CZs when you're jumping around, look at system map, see which factions are at war and if it's your faction against independent, bookmark it.
