The usual go without saying, google maps, banking, whatsapp etc.
My top 6 additionals are
1. Agar.io (My favourite game at present)
2. Keep (synchronised note heaven)
3. Quickpic (Much much better gallery app)
4. Manual camera (Control shutter speed on your phone!)
5. Smart remote (Worth every penny, control your appliances with your phone)
6. Sygic speed cameras (Overlays a speedcamera warning on google maps)