The colonies listen as "open" in the first post are all significant, with breathable atmospheres and abundant natural resources. I'd recommend Brazillian Amazonas, Israeli Zion, Canadian Elizabeth, Indian Dharma, Korean Gyeongju, or Indonesian Merdeka, depending on what culture you prefer. Federate Babylon is also a choice, though it lacks a specific national identity. Chinese 192-209 is a good system, though it's sort of dwarfed by the other Chinese colonies, and the PRC doens't give its governors a whole lot of leeway. Russian Zenit and Japanese Yamato are both colonies of the highest quality, so you can sign up for them if you're willing to take on that responsibility. Finally, American Liberty is listed as open, but I expect that Symphony would prefer not to have a player governor there until he's dealt with the native sapients to his liking, and you'll probably get vetoed if you try to sign up for it.