Aaron gave the best feedback for sure
Core, KOTOR, Scum & Villainy.
I still like Threats of the Galaxy as it adds the element of say a "Monster Manual" if you will for the GM. The main books do add a bunch of this information specific to the era/topic they cover. Threats is more broadbased.
Scavengers Guide to Droids and Starships are both specific to those areas and are not recommended unless you plan to go down those roads.
I really liked Unknown Regions and Galaxy of Intrigue, especially GoI. I found that it included a lot of options for new players (Scenarios and a Campaign at the end) and it covered the "darker" side of the galaxy reminding us that not every campaign needs to focus around core galactic events. I dunno, I found this book really solid.
Galaxy at War is still a solid book too though as well so I suppose.
Also, some I have done reviews on The HOLOCRON (there are 2 waiting to be proofed) but here is a link to the forum anyways:
http://www.the-holocron.com/index.php?o ... &Itemid=85 . I only have a few up (2 still need to be proofed).