In a base defense role, I would choose the Nano-Armor Accelerator suit certification. The Nano-Armor Accelerator certification grants armor repair while out of combat. This is an ideal ability when defending a base because you will be able to self-heal your armor between fights with an assault force. As a MAX, you have the ability to take out infantry relatively quickly, and using this armor to recover between engagements will make a huge difference.
The weapon loadout I would choose for base defense is the Falcon/Scattercannon for New Conglomerate, M1 Heavy Cycler/M3 Punder HEG for Terran Republic, and the Comet/Quasar for Vanu Sovereignty. I would choose this loadout because it gives the MAX flexibility to fight effectively against either vehicles or infantry. In addition to this cert, I would take the Laser Sight cert and attach it to the rail of the weapon to increase accuracy when firing from the hip.