Tatooine, Kashyyyk, Manaan, Korriban
It's what Wookieepedia's articles assume is the order the player chooses.
I think it's the recommended order in terms of overall difficulty and story progression.
It makes the most practical sense because the final two party members are on Tatooine and Kashyyyk so they should be first anyway, and also because you can't take Bastila on Korriban, yet if you save it for last, she's not with you anyway.
It makes more sense to me for the Leviathan to capture you when leaving Manaan as it would be no surprise for a Sith Interdictor to be in that system, whereas it wouldn't make sense for it to be at Tatooine or Kashyyyk without attracting unwanted attention.
It's also the order Bastila lists them in the "revelation" scene....I think?