For me the must-have are:
- Democratic Detonation (everything)
- Freedom's Flame (fire resist armors and cookout)
- Servants of Freedom (ultimatum and hellbomb)
Then optionally:
- Dust Devils (elemental resist armors, coyote and harpoon)
- Control Group (warp pack, epoch)
- Urban Legend (siege-ready armors, AT emplacement)