This is probably pretty sad on my part, but the app that I use most aside from Facebook, Instagram, and the camera, is actually my flashlight app. You never know when you might need a flashlight. I use it at night when I take the dog out or if I drop something, and it comes in handy when there's a power outage.
I use the google map app a lot. It's so great to be able to look up something wherever you are. I also love the 'Stay at home Mum' app, always some interesting stuff to read while you wait in line at the supermarket. I do most of my shopping through the Woolworths app so that would be another favourite.