I've always thought that the main planet is the one in the habitable zone (the one that can have liquid water), or the most inhabitable one if there are many (usually, because some are GG satellites).
Actually, you need to think of the question more from a standpoint of colonization ... because (almost) ALL star systems on the (hex) maps are colonized from interstellar sources, aside from the odd major/minor race out there with a homeworld.
Viewed from a standpoint of colonization, there are going to be two criteria to meet for colonization efforts to be "successful" over the long term:
- Habitability
- Ease of access to resources (matter, energy, food, fuel, biosphere, etc.)
Point 1 can be "solved" with sufficient investment (capital) expenditure combined with sufficient technology (in most cases, TL=7+ for nuclear power). The easiest solutions to the point of habitability are "ripe for colonization" worlds that have shirtsleeves environments (your basic variety Garden World).
Point 2 can be "solved" by a richness of exploitable resources for export, particularly during the "boot up" phase of a campaign of colonization. Once a world reaches Population: 7+ it can become more or less self-sustaining, economically, although it may still have "deficiencies" that make the world a reliable importer of goods (see: Desert, Industrial, Non-agricultural, Poor trade codes).
These two factors essentially "conspire" together in a way that can bias colonization efforts towards worlds that are expected to become Rich and/or Agricultural (with the right amount of Population and type of Government). Everywhere else that is Population: 6- (and especially 4-) is basically left "fighting for scraps" in the colonization game of trying to attract immigrants (in addition to nativist baby booms) so as to reach Population: 7+ and move from a resource extraction (for export) economy to a more balanced economy for domestic production and consumption (while still retaining some export/import potential).
Therefore, the "jobs" in a colonization effort (and therefore the people needed to DO those jobs) will tend to gravitate towards the location where it is "cheapest" to achieve the goals of that colonization.
If there's a shirtsleeves environment Garden World ... go there. Never underestimate the draw of a land of milk and honey!
If there's a "resource rich" planet/moon/belt that can be exploited easily ... go there. Think boom/bust mining towns for an historical example.
However, not EVERY world is ideally suited for colonization.
Some worlds have a history of "failed" attempts at colonization.
Some worlds already went through the "boom" times and are now sliding down the "bust" end of the economic cycle, as the demand for work and workers dries up and the population emigrates away to "better pastures" (if they can).
And some places were strip mined for everything they had and are now in the process of being abandoned/left stranded, because there's nothing left there to exploit (economically) ... so the "money" left.