Runners companion has alot of cool stuff. Most noteably new metatype variants & new positive and negative qualities
Arsenal is packed to the brim with gear, drones, and vehicles. It also has martial arts towards the back, which is probably my favorite thing about it.
Core Rulebook has all the basic gear, qualities, spells, and everything else you need to know about. I would highly recommend reading all of it when you get the chance.
I use those three the most, however, I've also found Augmentation and Runner Havens to be useful.
Augmentation is about all kinds of medical stuff. Mostly enhancements. The book is organized really strangely, but if you skim through it you can find some pretty crazy stuff.
Runner Havens is a supplement that includes alot of great information on Seattle in shadowrun. Also has info on some other cities; most notably Hong Kong (I think).
Books I need:
Street Magic
Seattle 2072
SR4A Core Rulebook
Unwired
I'd also like to get whatever book has the info on Neo Tokyo. Ooo and that new Almanac. World map is gonna be really nice to have.