population is one metric, as it resolves to income.  However, specialized planets with lower population might be better, especially if further away from your source goods.

Thank you for your answers. Income seems to be the consensus along with the distance factor.

That's good, because I have just found a Q25 planet that loves to make money and sits far away from anybody else 

you also get a diplomacy bonus for having a trade route, so sometimes it can be worthwhile to send them races you have differing ideologies from just to help offset the negatives.

Thanks Charch,

"you also get a diplomacy bonus for having a trade route, so sometimes it can be worthwhile to send them races you have differing ideologies from just to help offset the negatives."

You know, that's the way it used to be, but I no longer see that listed as a plus in the Diplo pop-up when hovering over another Civ.

I still see things like Out of Travelling Range, Ripe for Conquering, Open Borders Treaty, etc, but no more mention of the Trade Routes. I don't know what's up with that.

There was a change in GC3 at some point, maybe Crusade only, where the diplomatic bonus from trade only applies to civs that have certain trading/economic traits.  I'm not clear on which civs have that bonus and which don't, although I think that the Iridium definitely have it.

Hi Charch,

Maybe you're thinking about the Open Borders Treaty? That still works.

That, I did not know. Thanks for the tip!