I forgot to add as a good sector: Trojan Reach. It has everything that the Spinward Marches has, but has less "canon" development so is a little more open. For a long time, my players were based out of Bendor and/or Berengaria. Trojan Reaches have Aslan, Imperials, Zhodani, Vargr, some Solomani, some other races.
Far Frontiers, nice sector, but perhaps a little too far away from the Imperium.
My non-canon Reavers Deep sector with New Texas, Friedland, Sutekh, Bendor, etc. Wild and wooly, wide open adventure.