My top recommendations would be the original boxed set and the Rock of Bral, with the Monstrous Compendia, Astromundi Cluster, and maybe the Complete Spacefarer's Handbook on tier 2.
Lots of other products are mineable for stealable bits, but IMO just bits.
And then I just really dis-recommend the Realmspace supplement, which I view as a mix of bad writing, blatant misunderstandings of Realmslore (the author named a major T'u Lung city as the Shou Lung spaceport, for example), bad mechanics and math, and excesive dwelling on useless things (Eliminster's Safehold, details about how the Batship is booby-trapped against players ever being able to make use of it, and so on).