If you could only buy ONE of these stuff packs (for $5) which would you pick and why?

I would pick Kids Stuff hands down. I own all the stuff packs and it's probably my favourite due to voidcritters. It gives nice kids specific stuff to do with the battle station and also the puppet theatre. I've been currently playing twin boys who are void critter fans and it's been fun. They have been collecting the cards, looking for them in the bins, trading with other kids, battling together, training alone (also autonomously). You must want to play kids for this pack to make sense.