Definitely the Revised Core book. Beyond that, it really depends on your game.
Hero's Guide can be good for everything, and its really got some great ideas in there. Galactic Campaign Guide is a good GM book. More alien species in Ultimate Alien Anthology are great if you want more variety, and Ultimate Adversaries provides some new pre-statted enemies for a GM. If you're able to find Starships of the Galaxy at a reasonable price, pick it up. It may not be revised, but its not hard to convert and its definitely worth it.
If you run a Jedi heavy game, I'd suggest Power of the Jedi and The Darkside Sourcebook.