Arsenal greatly increases the survivability and power of riggers and all personal combatants through an infusion of weapons, armor and vehicles, along with the rules to modify them all to gleaming goodness. Martial arts and the toxins and drugs are also very good additions to help people keep up with the mage.
Runner's Companion, despite its flaws is still a great source of character depth for people getting into the game. There are a lot of new options and qualities that can destabilize a game if abused, but which generally just add to the flavor. I'm a big fan of the "Advanced Lifestyles" in their more intricate look at life in the 6th world. Ever wondered just what it means to have a "Street" lifestyle? Well, it's there on the page. Great resources for Players and GMs to draw from.
Unwired might seem like it is only useful to Hackers, Riggers and Technomancers, but.. well.. it pretty much is. There is a lot of internal contradiction as to just what can be done with and in the matrix. Enough that lots of tables just outright handwave it away. I feel like this is a terrible thing though, because the Matrix is literally everywhere in the 4th ed setting. Without it, there's really something missing. Big utility of this book is for GMs to help develop their setting. Also, things like program options and commlink upgrades are useful to all the characters, so long as you're playing in a truly jacked-in world. Technomancers are made considerably more powerful by the rules included here.
Augmentation is the mundanes' best friend. Another step closer to non-mage characters being able to bust heads like a wizard. The section on biodrones, cyberzombies and other monstrosities are pretty good GM fodder. The advanced medicine rules are great fluff as well.
Hope this helps you figure out where your priorities are bound to take you first.
