I just bought the giant box set and was wondering what would be the best thing to purchase next? 

Both Toolkits, Adventure's and GM's; then the Creature Guide and Creature Vault.

But if you have wizards or priests, then you might also want to consider getting the Winds of Magic or Signs of Faith boxes after the Adventurer’s Toolkit. Creature Guide & Vault are best for you if you write your own adventures. If you prefer to play stuff produced by FFG then you don't necessarily need these particularly quickly.

I would suggest the following, in this order:

• Creature Vault

• Lure of Power

• Adventurer's Toolkit

• Signs of Faith, Winds of Magic, and/or Omens of War (depending on your party composition) 

SoF and WoM both get a lot of lip service around here about being purchased immediately, but while they both have some great material, LoP both have more useful material for everyone. 

The Creature Guide was a waste IMHO. You don't get a lot of info that you can't get elsewhere on the internet. The GM Toolkit was a waste at full price, but may be worth it were it deeply discounted.