Synthetic + Prolific + Xenophobic. Start with colony ship.
Getting pop early for synthetic is difficult as you literally have to build your pop. However, you don't have to worry about pop cap or farming or morale. Like you can put 8 billion people on a class 4 planet and it's perfectly fine as synthetic.
Prolific provides +50% population upon settling. So: put two colony modules on a colony ship (6 pop) and settle some random planet and get +3 pop just like that.
Rinse and repeat. As long as you have enough administrator points, which you can get by beelining the government techs after the early +1 movement techs, and you can keep on colonizing and never run out of pop even as synthetics.
For ideology, pick up benevolent for the free colony ship, and later the level 3 ability that provides an extra +2 pop upon colonization. With this trick you can out expand the AI even on Incredible and still have all of the bonuses of being synthetic.
You want Xenophobic for the massive social production and research boosts (building population as synthetic is social production).