Edit: So I've realized the tactic is even better because you get starting legions. I've been playing suboptimally, its even crazier than I thought. So removed the citizen note.
So a tactic I've been doing is this.
1) Scout out for any neutral forces in my area.
2) Research planetary invasion
3) Load the legions on a transport.
4) Go around and immediately take every neutral planet, suffering 0 resistance.
The tactic seems way too good. For a minimum of cost I get a Class 16 planet (most of the time) with infrastructure and population already formed. These planets are night and day better than anything else I can get, and don't cost administration. Further, I can just keep doing it. With 1 transport, I can take 1,2, or even 3 neutral worlds. I don't even need any ships.
I think the issue is that neutral planets have 0 defenses. No ships, and no garrisoned legions. I think it minimum they should have a garrisoned legion and a few ships...something that at least slows the tactic down a bit.
So far, this seems to be the absolute best way to play. I get such a crazy return from the tactic I am miles ahead of a player that doesn't do this.