Depends what you want to add to the game.
Matrix adds more depth to the matrix and decking(hacking).
Magic in The Shadows adds a lot to magic, including new spells, metamagic, and a whole lot more.
Rigger 3 revised adds a lot to vehicles, combat and building custom designs.
SOTA 2063, add a few things like bioware additions, added security, merc campaings and more magic.
Target Wastelands adds more campaing settings like the artic and the desert. Including gear for those harsh climates.
Target Awakened Lands also nick named Target australia, because the bulk of it s taken up by the land of Oz. Adds some flavor tothe world and some magic.
Shadow Companion it adds the point system an alternate way to make characters and new meta variants and I can't remember much else.
New Seattle well it covers Seattle after 63 I think.
Dragon of the Sixth World, well the title covers that.
Shadows of North America covers the major history of North America up to now, or 63
Sprawl Survival Guide, adds to the everyday life aspect of Shadowrun. Like housign costs and enterainment. General fluff of the sixth world.
Wake of the Comet and Year of the Comet revolves around Halley's comet comeig back on the 50th anniversery of the awakening and settingoff a bunch of stuff, like SURGE.
Theats 2 is just more plot hooks to use for campaings.
Depending on what you want to add to your game just look at
the Offical site and read about the stuff available.
You may want cars and big guns so Rigger 3 is what you'll want. If your group isn't to keen on hacking company computers then Matrix will be of no use to you.
Tell us what you want to play and we can tell you what books to buy. Some of us will say buy them all.